Create an empty `versions.txt` if it doesn't exist in `repository-path`

[JoeWang1127]

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.