Fix the adding of empty clauses

[Dekker1]

These should correctly signal that the problem has been made unsatisfiable

[codecov]

Codecov Report

Attention: Patch coverage is 64.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 75.61%. Comparing base (4a16611) to head (e9a239a).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
crates/pindakaas/src/lib.rs	25.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #73      +/-   ##
===========================================
+ Coverage    75.58%   75.61%   +0.02%     
===========================================
  Files           31       31              
  Lines         7431     7439       +8     
  Branches      7431     7439       +8     
===========================================
+ Hits          5617     5625       +8     
- Misses        1690     1691       +1     
+ Partials       124      123       -1     

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