fix(ModflowSfr2): support single reach models

[jlarsen-usgs]

• single reach models crashed due to np.diff(reach_data).max() > 0 comparison, fix for this issue breaks up this comparison by taking the diff first and evaluating if the diff returns an empty result prior to taking the max()

[jlarsen-usgs]

Testing to be added shortly

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.6%. Comparing base (556c088) to head (cce415a).
⚠️ Report is 226 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/modflow/mfsfr2.py	82.3%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2841      +/-   ##
===========================================
+ Coverage     55.5%    73.6%   +18.0%     
===========================================
  Files          644      659      +15     
  Lines       124135   132282    +8147     
===========================================
+ Hits         68947    97402   +28455     
+ Misses       55188    34880   -20308     

Files with missing lines	Coverage Δ
flopy/modflow/mfsfr2.py	85.5% <82.3%> (-1.8%)	⬇️

... and 585 files with indirect coverage changes

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