Improve load times by skipping serialization of entities when unecessary. (backport #2596)

[mergify[bot]]

🦟 Bug fix

Fixes #

Summary

I was investigating https://github.com/gazebosim/gazebo_test_cases/issues/1576 , in my investigation it came to my notice that sdf::Root takes a long time to be constructed. While I am submitting changes upstream to reduce the impact of the creation of sdf::Root, in the event that we don't serialize a model, I'm proposing we just send an empty string accross. This should minimize both network traffic and make load times more manageable.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [x] 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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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This is an automatic backport of pull request #2596 done by Mergify.

[mergify[bot]]

Cherry-pick of 1a881310cfe2e1c97bdb7d606cf973eebc17a6c1 has failed:

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    both modified:   include/gz/sim/components/Model.hh

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