fix: trap grb in MF6Output

[wpbonelli]

Noticed and fixed by @jlarsen-usgs in e8118b8, just getting this in asap to fix CI tests.

This is a result of MODFLOW-ORG/modflow6#2228

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.8%. Comparing base (bb9824e) to head (f0dcfca).
Report is 66 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2468     +/-   ##
=========================================
+ Coverage     68.4%   75.8%   +7.4%     
=========================================
  Files          294     293      -1     
  Lines        59390   61946   +2556     
=========================================
+ Hits         40652   47000   +6348     
+ Misses       18738   14946   -3792     

Files with missing lines	Coverage Δ
flopy/mf6/utils/output_util.py	79.0% <100.0%> (+6.6%)	⬆️

... and 254 files with indirect coverage changes

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