Check for 0 stddev when generating a random number using normal distribution

[iche033]

🦟 Bug fix

Fixes gazebosim/gz-sim#2519

Summary

We use std::normal_distribution in our Rand class. From the doc:

The behavior is undefined if stddev is not greater than zero.

So added a check for 0 stddev (sigma) and return mean if its 0.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 93.98%. Comparing base (11caaf7) to head (59848ab).
Report is 8 commits behind head on gz-math7.

Additional details and impacted files

@@             Coverage Diff              @@
##           gz-math7     #615      +/-   ##
============================================
- Coverage     94.11%   93.98%   -0.14%     
============================================
  Files           146      149       +3     
  Lines          9809     9956     +147     
============================================
+ Hits           9232     9357     +125     
- Misses          577      599      +22     

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