Remove undefined behavior from QMessageLogContext

[jnbooth]

According to the safety documentation for CStr::from_ptr, the pointer must be non-null. Currently, the functions for QMessageLogContext do not check for null pointers. This PR adds those checks, removing the undefined behavior.

Note: cxx-qt-lib's qtlogging code is noticeably messy. In addition to the above undefined behavior, it has extraneous unsafe specifiers and a pointless const assertion that was probably supposed to occur on the C++ side, and its functions are in the global namespace rather than rust::cxxqtlib1. It also solely exposes undocumented internal functionality and doesn't respect category disabling. I made #1398 to serve as an alternative for it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (55106f0) to head (771fd43).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1399   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines        13124     13124           
=========================================
  Hits         13124     13124           

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