Bugfix: VTK Output of empty partitions

[holke]

Describe your changes here:

Fixes #1953
We were not able to write empty partitions any more due to this call in the VTK writer:

grid_get_local_bounds (grid, bounds);

For an empty partition (i.e. no cells to write in VTK), the cmesh function t8_cmesh_get_local_bounding_box is called anyways and raises the assertion T8_ASSERT (num_local_trees > 0);

Fixed it by wrapping the line in an if (num_cells > 0).

This was a minimal fix, since i felt like not touching the rest of the writer code.

However, it seems like VTK is smart and does not even write a file for the empty processes.
Thus, we could also go a step further and skip all of the remaining code in the writer function if the partition is empty.
What do you think @Davknapp?

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[Davknapp]

Yes, sounds like a good idea! Do you want to do it in this PR, or in another one?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.66%. Comparing base (23dba8f) to head (e1ece4a).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1954      +/-   ##
==========================================
- Coverage   75.67%   75.66%   -0.01%     
==========================================
  Files         105      105              
  Lines       18648    18646       -2     
==========================================
- Hits        14111    14109       -2     
  Misses       4537     4537              

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