fix!: Ensure ArrowTypeString never returns NULL

[paleolimbot]

Previously an unknown type identifier would have resulted in a NULL; now a conspicuous-looking possibly helpful string is shown.

Almost all internal usage of ArrowTypeString is use in Printf strings, for which gcc now displays a warning if the value could be NULL.

This is a breaking change if anybody was relying on the null output here to detect an invalid type identifier.

Closes #884.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.59%. Comparing base (f11d517) to head (8d0859f).
⚠️ Report is 47 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nanoarrow/common/inline_types.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #885      +/-   ##
==========================================
- Coverage   79.96%   78.59%   -1.38%     
==========================================
  Files         105      106       +1     
  Lines       15461    16119     +658     
  Branches     1738     1858     +120     
==========================================
+ Hits        12364    12669     +305     
- Misses       1998     2283     +285     
- Partials     1099     1167      +68     

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