Gui.cc: return "" instead of nullptr

[scpeters]

🦟 Bug fix

Fixes return values in a function

Summary

The defaultGuiConfigFile function returns type std::string, so return an empty string instead of nullptr in error cases.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.85%. Comparing base (ab18888) to head (aa76af0).
Report is 150 commits behind head on gz-sim8.

Files with missing lines	Patch %	Lines
src/gui/Gui.cc	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-sim8    #2819      +/-   ##
===========================================
+ Coverage    65.75%   68.85%   +3.09%     
===========================================
  Files          327      340      +13     
  Lines        31233    33011    +1778     
===========================================
+ Hits         20537    22729    +2192     
+ Misses       10696    10282     -414     

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