New rule var-missing-data

[forman]

Added a new core rule var-missing-data that checks for the recommended use of a variable's missing data.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (8c16501) to head (25af807).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files          66       67    +1     
  Lines        2746     2773   +27     
  Branches      401      408    +7     
=======================================
+ Hits         2731     2758   +27     
  Partials       15       15           

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