Fix hanging P4est MPI simulation with boundary conditions

[benegee]

Removing the if n_boundaries > 0 check fixes #1878.

Nothing will be done on the rank with 0 boundaries but p4est_iterate and ultimately MPI AllReduce will be called by all ranks.

The other checks for the mortars seem to be likewise at risk to me.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.30%. Comparing base (909abb4) to head (feeb716).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1879      +/-   ##
==========================================
- Coverage   96.30%   96.30%   -0.00%     
==========================================
  Files         440      440              
  Lines       35793    35790       -3     
==========================================
- Hits        34470    34467       -3     
  Misses       1323     1323              

Flag	Coverage Δ
unittests	96.30% <ø> (-<0.01%)	⬇️

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