chore(librarian): resolve issue where version file would not be updated in proto-only library (#14675)

This is a fast-follow fix to #14674 to address an incorrect assumption that `Path().rglob` would be empty if no matches were found. We need to explicitly convert the result to a list and check the length of the results.

Author: parthea

.generator/cli.py

@@ -1092,8 +1092,8 @@ def _update_version_for_library(
  """
 
  # Find and update gapic_version.py files
- version_files = Path(f"{repo}/{path_to_library}").rglob("**/gapic_version.py")
- if not version_files:
+ version_files = list(Path(f"{repo}/{path_to_library}").rglob("**/gapic_version.py"))
+ if len(version_files) == 0:
  # Fallback to `pyproject.toml`` or `setup.py``. Proto-only libraries have
  # version information in `setup.py` or `pyproject.toml` instead of `gapic_version.py`.
  pyproject_toml = Path(f"{repo}/{path_to_library}/pyproject.toml")