Empty object generation when using maps and multiple files.

[sauldhernandez]

Hi! I started to use this library not too long ago, great work!

So, I ran into a strange duplicate class error when using the protobuf builders generator, and having option java_multiple_files = true, compilation would fail whenever I have a message with a mapped field:

Task :myapp:kaptKotlin FAILED
e: <path to app>/build/generated/source/proto/main/java/com/myapp/Mapped.java:9: error: duplicate class: com.myapp.Mapped
public  final class Mapped extends

I tracked down the error to a generated object Mapped with no elements. Looking into the code I realized that it was generated there, but the filter didn't take into account map definitions (as is in other places in the generator)

I tested it and seems to solve the issue, test pass, but I don't know if there's anything else I should change.

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[marcoferrer]

Thanks for the contribution, this was a great catch. Would you mind adding a test sample for the previously failing proto to the test api module? And a very basic unit test in generator tests module. Something as simple as calling the fixed builder. This way the compiler can catch any regressions