fix(model_splitter): fix spurious ssm creation

[wpbonelli]

fix #2715

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (41dc1c6).
⚠️ Report is 131 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2716      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      674      +30     
  Lines       124135   130548    +6413     
===========================================
+ Hits         68947    94595   +25648     
+ Misses       55188    35953   -19235     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	65.5% <ø> (-9.4%)	⬇️

... and 569 files with indirect coverage changes

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