Fix Protobuf .d.ts typings for .proto files without package

[Yannic]

Fixes #939

[Yannic]

/cc @uwemaurer

[stanley-cheung]

@uwemaurer Thanks for the report.
@Yannic Thanks for the fix.

I was wondering whether we can add some tests for this.

There were some tests involving stuff with no proto package before. See this test proto, and this test for trying to execute these relevant code path in the codegen plugin.

If this has to do with Dts generation and we need to test whether some generated code survives the tsc compilation, we have some proto like this, such that we have tests like this first to run the plugin to generate the generated code (which would exercise the code changed in for example this PR), and then run tsc on the generated code to make sure things are compiled properly.

[Yannic]

@stanley-cheung done! ptal, thanks.

[stanley-cheung]

Thanks for adding the test!