GaussianNoiseModel: avoid calling DblNormal with invalid standard deviation

[oysstu]

🦟 Bug fix

Fixes #395

Summary

The fix checks for zero standard deviation before sampling the normal distribution. If the standard deviation is zero, the mean is set directly. I've fixed both cases as mentioned in #395, as I think a crash is undesirable for both cases, but perhaps a warning should be emitted in Load() if the state standard deviation is zero.

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

Merging #396 (e95f6f2) into main (b834d76) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head e95f6f2 differs from pull request most recent head 1216651. Consider uploading reports for the commit 1216651 to get more accurate results

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
+ Coverage   72.34%   72.36%   +0.01%     
==========================================
  Files          37       37              
  Lines        5052     5055       +3     
==========================================
+ Hits         3655     3658       +3     
  Misses       1397     1397              

Files	Coverage Δ
src/GaussianNoiseModel.cc	98.38% <100.00%> (+0.08%)	⬆️

[iche033]

changes look good to me.

@scpeters, Perhaps we should add a check in gz::math::Rand::DblNormal to make sure standard deviations are greater than 0? As pointed out in https://stackoverflow.com/a/9186515:

explicit normal_distribution(RealType mean = 0.0, RealType stddev = 1.0);
Requires: 0 < stddev.