test: test for NULL string in string_append_char

[aloisklink]

Adds a test case for string_append_char when the input string is null.

[codecov]

Codecov Report

Merging #244 (8240aa4) into main (75dac55) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 8240aa4 differs from pull request most recent head c17e850. Consider uploading reports for the commit c17e850 to get more accurate results

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   45.61%   45.63%   +0.02%     
==========================================
  Files         107      107              
  Lines       14951    14953       +2     
==========================================
+ Hits         6820     6824       +4     
+ Misses       8131     8129       -2     

Impacted Files	Coverage Δ
tests/utils/test_os.c	98.03% <100.00%> (+0.01%)	⬆️
src/utils/os.c	40.25% <0.00%> (+0.25%)	⬆️

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[AshleySetter]

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