http-client-java, report TCGC diagnostics

[weidongxu-microsoft]

link Azure/typespec-azure#1487

tested with

Union U {
  null;
};

after this PR, we will see

C:/github/typespec/packages/http-client-java/generator/http-client-generator-test/main.tsp:15:7 - error @azure-tools/typespec-client-generator-core/union-null: Cannot have a union containing only null types.
> 15 | union U {
     |       ^

and codegen fails (as this is an error).

[azure-sdk]

No changes needing a change description found.

[azure-sdk]

You can try these changes here

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[weidongxu-microsoft]

CI failure due to (likely) a wrong lint in TCGC. They seems mistakenly flagged a JSON model as one used by both JSON and multipart/form-data
Azure/typespec-azure#1510

Need to wait for a fix in TCGC.