Do not interpret empty shape as None in RandomVariables

[ricardoV94]

Description

Marking this as major, because functionality elsewhere (PyMC) relied on this interpretation

Marking as draft as it includes first commit of #664

Related Issue

• [x] Closes #568

Checklist

• [ ] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [ ] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [x] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):