Fix setting initvals on ZeroSumTransform rv

[velochy]

Description

Fix setting initvals on ZeroSumTransform'ed rv

Related Issue

• [X] Closes #7772

Checklist

• [X] Checked that the pre-commit linting/style checks pass
• [X] 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

• [X] Bug fix

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📚 Documentation preview 📚: https://pymc--7773.org.readthedocs.build/en/7773/