Some MatrixRing fixes

[lgoettgens]

• Check n in the constructor to be non-negative. Resolves Prevent negative dimension matrix rings #1851.
• Fix characteristic(matrix_ring(R, 0)) to be 1.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.14%. Comparing base (c8aec15) to head (92a3a64).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1852      +/-   ##
==========================================
- Coverage   88.15%   88.14%   -0.01%     
==========================================
  Files         119      119              
  Lines       30017    30021       +4     
==========================================
+ Hits        26460    26462       +2     
- Misses       3557     3559       +2     

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