Fix Base.show StructuredMesh

[MarcoArtiano]

When the StructuredMesh was called without any mapping, the information was displayed incorrectly. See below.

Before

After

Same for 1D and 2D.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.95%. Comparing base (e93de2a) to head (bf38f93).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2228      +/-   ##
==========================================
- Coverage   96.42%   93.95%   -2.47%     
==========================================
  Files         487      487              
  Lines       39352    39345       -7     
==========================================
- Hits        37942    36964     -978     
- Misses       1410     2381     +971     

Flag	Coverage Δ
unittests	93.95% <100.00%> (-2.47%)	⬇️

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[MarcoArtiano]

Now, when no mapping is provided to StructuredMesh, Trixi displays the following information:

Analogously for 1D and 2D cases.

Fixed also when StructuredMesh is called with faces