Model version tag must be null on API creation

The endpoint CreateModelVersion must support either:

• a null tag
• a non-blank tag (like 1.2.3 or beta2...)

Currently it does not support null tags, which must be supported.

Please test locally with publish_models.py:

1. on empty model versions
2. on existing model versions with status created
3. on existing model versions with status available
4. on existing model versions with status error