Fix entity tree for large worlds

[chapulina]

🦟 Bug fix

Summary

Since #497 it's possible that plugins receive updates before any entity has been created. This happens for example for large worlds like tunnel.sdf, which take a while to load. This PR makes sure the tree keeps trying to get all entities until some are available.

To test it, run ign gazebo tunnel.sdf with and without this PR. Without this PR, the entity tree will be empty.

Checklist

• Signed all commits for DCO
• Added tests: no, tough to test large worlds that take time to load
• 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

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #673 (37d85dc) into ign-gazebo3 (7d98df4) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@               Coverage Diff               @@
##           ign-gazebo3     #673      +/-   ##
===============================================
- Coverage        77.88%   77.87%   -0.02%     
===============================================
  Files              210      210              
  Lines            11649    11650       +1     
===============================================
- Hits              9073     9072       -1     
- Misses            2576     2578       +2     

Impacted Files	Coverage Δ
src/gui/plugins/entity_tree/EntityTree.cc	13.11% <50.00%> (-0.63%)	⬇️

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