[sdk-platform-java] Remove/do not generate `pom.xml` if no source code exists in the module

We need to remove empty modules (except for `pom.xml`) when transport changes.

For example, in https://github.com/googleapis/googleapis/commit/de97a5f806a215960023d8d184a733260f8a9d3f, the transport changed from `grpc+rest` to `rest` and the source code in the grpc module is removed. However, the `pom.xml` in the module is not removed and cause test failure in graalvm build:
```
09:13:06:345 [ERROR] Failed to execute goal org.graalvm.buildtools:native-maven-plugin:0.10.3:test (test-native) on project google-cloud-connectgateway: Missing jar-file for com.google.api.grpc:grpc-google-cloud-connectgateway-v1:jar:0.6.0:test. Ensure that native-maven-plugin runs in package phase. -> [Help 1]
```

We should either
1. Remove the `pom.xml` from a module if there's no source code
2. Do not generate `pom.xml` if there's no source code (this is preferable)

part of b/372713264