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

[mjreno]

Intended to address issue #2057.

[codecov[bot]]

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.

:exclamation: 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

```diff @@ 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](https://app.codecov.io/gh/modflowpy/flopy/pull/2234?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [flopy/mf6/data/mffileaccess.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2234?src=pr&el=tree&filepath=flopy%2Fmf6%2Fdata%2Fmffileaccess.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvbWY2L2RhdGEvbWZmaWxlYWNjZXNzLnB5) | `64.3% <45.0%> (-10.9%)` | :arrow_down: | ... and [201 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2234/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)