Allow None override for version_base and ancestor_id in create_package_version

[zenibako]

If you attempt to override the preset version_base and ancestor_id options for the create_package_version task (for example, to bypass GitHub resolution in the release_2gp_beta flow), you will be blocked by errors (_get_base_version_number() will attempt to parse the value as a version number, and _resolve_ancestor_id() doesn't recognize the value).

This PR adds a new utils.process_none_arg function, and updates create_package_version to use this new function for version_base and ancestor_id.