Panic when rendering a nullable example #301

openapi: 3.1.0
components:
  schemas:
    Thing:
      type: object
      description: A nullable example.
      properties:
        target:
          nullable: true
          type: string
          enum:
            - staging
            - production
          example:

func TestDocument_Nullable_Example(t *testing.T) {
	data, _ := os.ReadFile("../../../test_specs/nullable-examples.openapi.yaml")
	info, _ := datamodel.ExtractSpecInfo(data)

	lowDoc, _ = lowv3.CreateDocumentFromConfig(info, datamodel.NewDocumentConfiguration())

	highDoc := NewDocument(lowDoc)
	rendered := highDoc.RenderWithIndention(2)

	if runtime.GOOS != "windows" {
		assert.Equal(t, string(data), strings.TrimSpace(string(rendered)))
	}

	rendered = highDoc.RenderWithIndention(4)
	if runtime.GOOS != "windows" {
		assert.NotEqual(t, string(data), strings.TrimSpace(string(rendered)))
	}
}

=== RUN   TestDocument_Nullable_Example
--- FAIL: TestDocument_Nullable_Example (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10439b798]

goroutine 35 [running]:
testing.tRunner.func1.2({0x1048a6580, 0x104d2a520})
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1631 +0x3c0
testing.tRunner.func1()
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1634 +0x57c
panic({0x1048a6580?, 0x104d2a520?})
	/opt/homebrew/opt/go/libexec/src/runtime/panic.go:770 +0xf0
gopkg.in/yaml%2ev3.handleErr(0x1400028fc38)
	/Users/thomasrooney/go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:294 +0x120
panic({0x1048a6580?, 0x104d2a520?})
	/opt/homebrew/opt/go/libexec/src/runtime/panic.go:770 +0xf0
gopkg.in/yaml%2ev3.handleErr(0x1400028a228)
	/Users/thomasrooney/go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:294 +0x120
panic({0x1048a6580?, 0x104d2a520?})
	/opt/homebrew/opt/go/libexec/src/runtime/panic.go:770 +0xf0
gopkg.in/yaml%2ev3.(*Node).Encode(0x14000219220, {0x104905d60, 0x140001be780})
	/Users/thomasrooney/go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:269 +0x208
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).AddYAMLNode(0x14000012320, 0x14000218a00, 0x140002009c0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:505 +0x1124
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).Render(0x14000012320)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:256 +0x2f0
github.com/pb33f/libopenapi/datamodel/high/base.(*SchemaProxy).MarshalYAML(0x140002081c0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/base/schema_proxy.go:191 +0x190
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).AddYAMLNode(0x140000121e0, 0x140002188c0, 0x14000200840)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:449 +0xacc
github.com/pb33f/libopenapi/orderedmap.(*Map[...]).ToYamlNode(0x1049b88c0, {0x1049a87a0, 0x140000121e0}, {0x104969ce0, 0x14000208200})
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/orderedmap/builder.go:78 +0x70c
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).AddYAMLNode(0x140000121e0, 0x14000218500, 0x14000200780)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:427 +0x668
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).Render(0x140000121e0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:256 +0x2f0
github.com/pb33f/libopenapi/datamodel/high/base.(*SchemaProxy).MarshalYAML(0x14000208080)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/base/schema_proxy.go:191 +0x190
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).AddYAMLNode(0x140000120f0, 0x140002183c0, 0x14000200660)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:449 +0xacc
github.com/pb33f/libopenapi/orderedmap.(*Map[...]).ToYamlNode(0x1049b88c0, {0x1049a87a0, 0x140000120f0}, {0x104969ce0, 0x14000208100})
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/orderedmap/builder.go:78 +0x70c
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).AddYAMLNode(0x140000120f0, 0x14000218280, 0x14000200600)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:427 +0x668
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).Render(0x140000120f0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:256 +0x2f0
github.com/pb33f/libopenapi/datamodel/high/v3.(*Components).MarshalYAML(0x140000ae060)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/v3/components.go:158 +0x58
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).AddYAMLNode(0x14000012050, 0x14000218000, 0x140002005a0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:449 +0xacc
github.com/pb33f/libopenapi/datamodel/high.(*NodeBuilder).Render(0x14000012050)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/node_builder.go:256 +0x2f0
github.com/pb33f/libopenapi/datamodel/high/v3.(*Document).MarshalYAML(0x140000da0b0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/v3/document.go:189 +0x58
gopkg.in/yaml%2ev3.(*encoder).marshal(0x1400020c008, {0x0, 0x0}, {0x104905820, 0x140000da0b0, 0x16})
	/Users/thomasrooney/go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/encode.go:140 +0x5c8
gopkg.in/yaml%2ev3.(*encoder).marshalDoc(0x1400020c008, {0x0, 0x0}, {0x104905820, 0x140000da0b0, 0x16})
	/Users/thomasrooney/go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/encode.go:105 +0x174
gopkg.in/yaml%2ev3.(*Encoder).Encode(0x14000064010, {0x104905820, 0x140000da0b0})
	/Users/thomasrooney/go/pkg/mod/gopkg.in/yaml.v3@v3.0.1/yaml.go:251 +0xdc
github.com/pb33f/libopenapi/datamodel/high/v3.(*Document).RenderWithIndention(0x140000da0b0, 0x2)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/v3/document.go:169 +0xa8
github.com/pb33f/libopenapi/datamodel/high/v3.TestDocument_Nullable_Example(0x1400012a9c0)
	/Users/thomasrooney/Code/speakeasy-api/libopenapi/datamodel/high/v3/document_test.go:645 +0x160
testing.tRunner(0x1400012a9c0, 0x1049a3350)
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1689 +0x1a8
created by testing.(*T).Run in goroutine 1
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1742 +0x668```