Fix conditional import of zarr

[lucianopaz]

Description

Related Issue

• Closes Zarr is not optional #7644
• Related to #

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
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

---

📚 Documentation preview 📚: https://pymc--7645.org.readthedocs.build/en/7645/

[codecov]

Codecov Report

Attention: Patch coverage is 63.63636% with 8 lines in your changes missing coverage. Please review.

Project coverage is 92.77%. Comparing base (671d704) to head (ac9ef0a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc/backends/zarr.py	66.66%	6 Missing ⚠️
pymc/sampling/mcmc.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7645      +/-   ##
==========================================
+ Coverage   87.41%   92.77%   +5.36%     
==========================================
  Files         107      107              
  Lines       18168    18178      +10     
==========================================
+ Hits        15881    16865     +984     
+ Misses       2287     1313     -974     

Files with missing lines	Coverage Δ
pymc/sampling/mcmc.py	87.14% <50.00%> (-0.31%)	⬇️
pymc/backends/zarr.py	93.81% <66.66%> (-1.86%)	⬇️

... and 9 files with indirect coverage changes

[ricardoV94]

Putting no release notes as we haven't released yet and it would look confusing in the release notes