fix(model_splitter): fix spurious ssm creation

[wpbonelli]

The model splitter generated empty SSM package files for partitions containing no boundary conditions, causing MODFLOW 6 to fail.

The _remap_ssm() method set a dict entry to None to indicate a partition had no boundary flows. Later the dict is checked in an if statement and considered "truthy" since it contains an entry whose value is None

Instead, indicate no boundary flows by not adding an entry to the dict. The conditional works, no SSM package is created, no file written, MF6 is happy.

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 132 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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[jlarsen-usgs]

@wpbonelli, this looks good!