http-client-java, report TCGC diagnostics

[weidongxu-microsoft]

link https://github.com/Azure/typespec-azure/issues/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).