fix(model_attributes_to_shapefile): use modelgrid kwarg if provided

[wpbonelli]

The model_attributes_to_shapefile() function is described as accepting a "modelgrid" kwarg, but when provided it went unused. Pop it from kwargs, call to_geodataframe() on it if provided to build an initial gdf, then pass that as an initial gdf to the model's to_geodataframe().

Also, add a trap for base Grid in model-level .to_geodataframe() methods. The base Grid.to_geodataframe() method requires a features param since the grid contains no geometry info.

Related to #2744

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (58fdd83).
⚠️ Report is 171 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/export/shapefile_utils.py	60.0%	2 Missing ⚠️
flopy/mbase.py	80.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2768      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132714    +8579     
===========================================
+ Hits         68947    96319   +27372     
+ Misses       55188    36395   -18793     

Files with missing lines	Coverage Δ
flopy/mf6/mfmodel.py	57.6% <100.0%> (-23.3%)	⬇️
flopy/mbase.py	70.4% <80.0%> (-2.3%)	⬇️
flopy/export/shapefile_utils.py	58.6% <60.0%> (-25.4%)	⬇️

... and 577 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.