fix: handle empty protobuf package

[kilimnik]

See #273, the generators don't work correctly on empty package names. I fixed that by checking if the package name is empty and using it when needed.

[stephenh]

@dk99 thanks for the PR!

Can you add a new integration-test for this, i.e. make a new directory in integration/ with a name like no-proto-package, and put a ~as-minimal-as-possible proto file that reproduces your issue?

Then include the generated output in the PR, to both review + function as a regression test.

Thanks!

[kilimnik]

I have added one minimal integration test. I am unsure if that test covers all possibilities. To cover all possibilities it would be necessary to also compile this test for grpc-web.

[stephenh]

Thanks @dk99 ! I wanted to make a small tweak and haven't learned how to push to existing PRs, so I created #285 and will release from there. Thanks!