NETSDK1206 warns on platforms that should be vaild when project has no `FrameworkReference`

[elinor-fung]

Describe the bug

NETSDK1206 warning is firing when project has no FrameworkReferences. The warning uses determines valid/portable platforms based on processing FrameworkReference.

This can happen when DisableImplicitFrameworkReferencs is set and no framework references are added. This is likely not much of a problem for normal builds, but it is for building the runtime framework libraries themselves. We should probably not warn if there are no framework references.

To Reproduce

See dotnet/runtime#88827 - build of System.Net.Quic for windows or osx