fix: Eliminate warnings for gcc16 and _FORTIFY_SOURCE=3 for snprintf

[paleolimbot]

The ArrowSchemaToString() function used snprintf(NULL, 0, "...") to calculate final buffer sizes; however, this led to warnings (either because all NULLs are bad in this case or because the compiler couldn't prove that n was zero). This led to CRAN warnings for nanoarrow and all packages that vendored it because CRAN compiles on gcc16. Opus 4.5 seemed to suggest _FORTIFY_SOURCE=3 rejected all null inputs to snprintf, so this PR removes them.

In the process I tidied up the printing a bit since it was hard to follow.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.59%. Comparing base (f11d517) to head (9961883).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
- Coverage   79.96%   78.59%   -1.38%     
==========================================
  Files         105      106       +1     
  Lines       15461    16128     +667     
  Branches     1738     1858     +120     
==========================================
+ Hits        12364    12675     +311     
- Misses       1998     2286     +288     
- Partials     1099     1167      +68     

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