[ENH] allow returning empty variable lists in funcs

[solegalli]

I am changing the functionality of the variable that finds numerical and categorical variables when the user enters a list. This might have knock-on effects downstream on other transformers, particularly on the selectors that use the prediction module. Need to double check and update the documentation when suitable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (ec04aa4) to head (dde61c3).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #894   +/-   ##
=======================================
  Coverage   98.22%   98.23%           
=======================================
  Files         113      113           
  Lines        4846     4867   +21     
  Branches      770      777    +7     
=======================================
+ Hits         4760     4781   +21     
  Misses         55       55           
  Partials       31       31           

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