refactor: make None imply no selected wells

[tlambert03]

this makes all "falsey" values to selected_wells in the WellPlatePlan imply no selected wells (rather than all selected wells)

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.64%. Comparing base (5bf36c6) to head (bd4dc40).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/useq/_plate.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
- Coverage   93.78%   93.64%   -0.14%     
==========================================
  Files          18       18              
  Lines        1222     1228       +6     
==========================================
+ Hits         1146     1150       +4     
- Misses         76       78       +2     

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