More nullptr checking on visuals and base axis

[chapulina]

🦟 Bug fix

Needed by gazebosim/gz-sim#1105

Summary

A couple of these nullptr checks were needed by gazebosim/gz-sim#1105. While at it, I also went over Ogre2Visual and OgreVisual, added more nullptr checks, and reduced the diff between them.

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)
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[codecov]

Codecov Report

Merging #486 (7fcfd45) into ign-rendering6 (056023a) will decrease coverage by 0.01%.
The diff coverage is 35.48%.

@@                Coverage Diff                 @@
##           ign-rendering6     #486      +/-   ##
==================================================
- Coverage           53.60%   53.58%   -0.02%     
==================================================
  Files                 194      194              
  Lines               19645    19669      +24     
==================================================
+ Hits                10530    10539       +9     
- Misses               9115     9130      +15     

Impacted Files	Coverage Δ
ogre/src/OgreVisual.cc	22.96% <13.33%> (-0.43%)	⬇️
ogre2/src/Ogre2Visual.cc	84.96% <50.00%> (-4.30%)	⬇️
include/ignition/rendering/base/BaseAxisVisual.hh	94.82% <100.00%> (+0.09%)	⬆️
...e/ignition/rendering/base/BaseGaussianNoisePass.hh	100.00% <0.00%> (+3.33%)	⬆️

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[iche033]

looks good to me.

[chapulina]

Ticketed #488 for the ABI checker failure, it's not caused by this PR.