don't create components for entities that don't exist

[adlarkin]

Signed-off-by: Ashton Larkin [email protected]

🦟 Bug fix

Summary

Addresses an issue found in #913 where components would be created for entities that don't exist (see #913 for more information). I've also updated the diff_drive system plugin to make sure that it doesn't try to create components for entities that have been removed.

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 #927 (91e0d27) into ign-gazebo3 (34ac465) will increase coverage by 0.04%.
The diff coverage is 81.81%.

@@               Coverage Diff               @@
##           ign-gazebo3     #927      +/-   ##
===============================================
+ Coverage        77.87%   77.92%   +0.04%     
===============================================
  Files              215      215              
  Lines            12080    12089       +9     
===============================================
+ Hits              9407     9420      +13     
+ Misses            2673     2669       -4     

Impacted Files	Coverage Δ
src/systems/diff_drive/DiffDrive.cc	90.95% <66.66%> (-0.85%)	⬇️
src/EntityComponentManager.cc	84.50% <100.00%> (+0.14%)	⬆️
src/SimulationRunner.cc	94.42% <0.00%> (+1.07%)	⬆️

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[chapulina]

The fix here looks reasonable, but I think the problem in the diff drive is a symptom of a deeper problem: plugins aren't unloaded when a model is deleted: #113