fix(evt): optional field mishandling

[mjreno]

Address some mishandling of EVT list input optional fields:

• bug: expected fields in evt binary file #2057
• bug: Flopy does not load an MF6 EVT package with a Boundary Name #2353

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.1%. Comparing base (e2af433) to head (674f343).
Report is 11 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2490   +/-   ##
=======================================
  Coverage     76.0%   76.1%           
=======================================
  Files          296     296           
  Lines        64158   64205   +47     
=======================================
+ Hits         48803   48865   +62     
+ Misses       15355   15340   -15     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdatastorage.py	75.4% <100.0%> (-0.1%)	⬇️
flopy/mf6/data/mfdatautil.py	77.9% <ø> (ø)
flopy/mf6/data/mffileaccess.py	76.6% <100.0%> (+0.7%)	⬆️

... and 8 files with indirect coverage changes

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