Improvement: avoid segfault when passing a nullptr to t8_forest_element_face_neighbor

[dutkalex]

Describe your changes here:
The last argument of t8_forest_element_face_neighbor is an output parameter which gets filled with the dual face index. In some situations, this additional output is not desired and one would like to pass nullptr (as is often done in other t8code APIs) to signal this. Currently, this results in a hard crash. This PR enables this use case.

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.59%. Comparing base (0540e17) to head (58d1f3d).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
src/t8_forest/t8_forest.cxx	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2214      +/-   ##
==========================================
- Coverage   82.59%   82.59%   -0.01%     
==========================================
  Files         115      115              
  Lines       18545    18548       +3     
==========================================
+ Hits        15318    15320       +2     
- Misses       3227     3228       +1     

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[holke]

Thank you for this addition!
This is currently on hold since it conflicts with our large changes in #1299. But i will see how we integrate both of these.

[benegee]

Thanks for your suggestion!

With #1736 merged, this is still possible from my point of view.

[benegee]

I was first uncertain about this dummy. After looking through the code it is clear the variable is required locally. It is still an output, and if a user does not actually need it, it seems ok to just discard it.

[dutkalex]

This is the simplest and least intrusive change to allow for nullptr to be passed to this function. Conceptually, it boils down to moving inside of the function the call site boilerplate that callers are doing right now if they don't care about neigh_face. Reading your comment it is not 100% clear to me if you agree with this implementation or if you want me to change something however.

[benegee]

Checked this. I think the TODO can be removed.

[dutkalex]

I don't quite understand what this TODO comment is referring to. Is it related to the change implemented in this PR?