1325 invalid way of calling first element of pdseries

[wd60622]

Description

This uses iloc over empty bracket slicing

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1433.org.readthedocs.build/en/1433/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.55%. Comparing base (d9f0c51) to head (78bcfb1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1433      +/-   ##
==========================================
- Coverage   94.62%   94.55%   -0.08%     
==========================================
  Files          50       50              
  Lines        5416     5416              
==========================================
- Hits         5125     5121       -4     
- Misses        291      295       +4     

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