feat: get first cell with value other than `None`

[wastedareas]

Closes #799

Summary of Changes

Added static method to Cell, which returns a Cell containing the polars expression of the first Cell in the list that is not None. Works on a list of Cells, regardless of Cell type (numeric, string, date, etc.). Returns None if given an empty list.

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	2	0	0	1.05s
✅ PYTHON	mypy	2		0	2.96s
✅ PYTHON	ruff	2	0	0	0.36s
✅ REPOSITORY	git_diff	yes		no	0.6s

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.13%. Comparing base (9880fe0) to head (ebbdc54). Report is 55 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #904 +/- ## ========================================== + Coverage 97.63% 98.13% +0.49% ========================================== Files 117 122 +5 Lines 6057 6900 +843 ========================================== + Hits 5914 6771 +857 + Misses 143 129 -14 ```

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[wastedareas]

Do I need to resolve the conversations or will they be resolved by the reviewers? Because all the requested changes should be implemented in the latest commit.

[lars-reimann]

Yes, please resolve conversations you've handled yourself.

[lars-reimann]

:tada: This PR is included in version 0.27.0 :tada:

The release is available on:

• v0.27.0
• GitHub release

Your semantic-release bot :package::rocket: