Avoid nullptr access in MinimalScene

[chapulina]

🦟 Bug fix

Summary

While debugging a problem in ign-rendering, I noticed that we may access a null scene here. So just guard against that.

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 #380 (5fcff84) into ign-gui6 (ec910ed) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##           ign-gui6     #380      +/-   ##
============================================
- Coverage     65.59%   65.57%   -0.02%     
============================================
  Files            39       39              
  Lines          5139     5142       +3     
============================================
+ Hits           3371     3372       +1     
- Misses         1768     1770       +2     

Impacted Files	Coverage Δ
src/plugins/minimal_scene/MinimalScene.cc	64.03% <33.33%> (-0.17%)	⬇️

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