[v3-1-test] BUGFIX: trigger ui parameter field is dict when param.value is null (#58682)

* return 'null' when param.value is null, rather than typeof(null)

* added 'null' to list of enum types
(cherry picked from commit 0adfc5b)

Co-authored-by: Tomi <74303735+Tomi-1997@users.noreply.github.com>

Author: Tomi-1997

airflow-core/src/airflow/ui/src/components/FlexibleForm/FieldSelector.tsx

@@ -47,6 +47,11 @@ const inferType = (param: ParamSpec) => {
     return "array";
   }
 
+  // Missing value, return 'null' as typeof(null) = 'dict'
+  if (param.value === null) {
+    return "null";
+  }
+
   return typeof param.value;
 };
 
@@ -61,7 +66,7 @@ const isFieldDate = (fieldType: string, fieldSchema: ParamSchema) =>
 const isFieldDateTime = (fieldType: string, fieldSchema: ParamSchema) =>
   fieldType === "string" && fieldSchema.format === "date-time";
 
-const enumTypes = ["string", "number", "integer"];
+const enumTypes = ["null", "string", "number", "integer"];
 
 const isFieldDropdown = (fieldType: string, fieldSchema: ParamSchema) =>
   enumTypes.includes(fieldType) && Array.isArray(fieldSchema.enum);