fix(binaryheader): account for optional fields in evt spd binary header

[mjreno]

Intended to address issue #2057.

[codecov]

Codecov Report

Attention: Patch coverage is 45.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 35.8%. Comparing base (18dfcb0) to head (5f680d7).
Report is 3 commits behind head on develop.

❗ Current head 5f680d7 differs from pull request most recent head a01aa3e

Please upload reports for the commit a01aa3e to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #2234      +/-   ##
==========================================
- Coverage     73.5%   35.8%   -37.8%     
==========================================
  Files          294     294              
  Lines        58888   58901      +13     
==========================================
- Hits         43329   21122   -22207     
- Misses       15559   37779   +22220     

Files	Coverage Δ
flopy/mf6/data/mffileaccess.py	64.3% <45.0%> (-10.9%)	⬇️

... and 201 files with indirect coverage changes