feat: warn and exclude lights with no flats, fix prepare command when no filter used

[TomyCesaille]

Description

• Warn if there's lights with no flats matching
• exclude them, if any, from the rest of the pipeline (poto-siril is pointless without flats)
• bug fix a few stuff around layers with no filter
  • improve e2e test coverage around this

Related Issue

Closes #41

Type of change

• Chore
• Bug fix
• New feature
• Feature change
• Breaking change
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.81%. Comparing base (a637bb0) to head (135089e).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   94.33%   94.81%   +0.48%     
==========================================
  Files           7        7              
  Lines         600      617      +17     
  Branches      151      162      +11     
==========================================
+ Hits          566      585      +19     
+ Misses         33       31       -2     
  Partials        1        1              

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