Check if Distortion / noise render pass is null before setting it

[iche033]

🦟 Bug fix

The distortion / noise model pointer returned could be null because ign-rendering does not support it.

For example, distortion is supported in ogre 1.x but not ogre 2.x. So when ign-gazebo tries to load an ogre 2.x scene with camera distortion, it'll lead to a crash

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #211 (edd0079) into ign-sensors6 (836c306) will decrease coverage by 0.15%.
The diff coverage is 12.50%.

@@               Coverage Diff                @@
##           ign-sensors6     #211      +/-   ##
================================================
- Coverage         73.06%   72.91%   -0.16%     
================================================
  Files                33       33              
  Lines              3067     3075       +8     
================================================
+ Hits               2241     2242       +1     
- Misses              826      833       +7     

Impacted Files	Coverage Δ
src/ImageBrownDistortionModel.cc	0.00% <0.00%> (ø)
src/ImageGaussianNoiseModel.cc	73.52% <25.00%> (-6.48%)	⬇️

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-04-13-fortress-edifice/1367/1