[sdk-platform-java] Create an empty `versions.txt` if it doesn't exist in `repository-path`

In the hermetic library generation process, we require a `versions.txt` file exists in `repository-path`; otherwise raise `FileNotFoundError` in https://github.com/googleapis/sdk-platform-java/blob/7d6d66a161db5edc538aec065405954acf4434c5/hermetic_build/library_generation/utils/utilities.py#L164

We didn't check the content of this file and the post processor can add versions for new client, therefore we should create an empty file if not exist, rather than raise an exception.

The generated library will have a `0.0.1-SNAPSHOT` version.