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

[mergify]

🦟 Bug fix

Fixes #

Summary

I was investigating
gazebosim/gazebo_test_cases#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

• 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)
• 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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This is an automatic backport of pull request #2596 done by [Mergify](https://mergify.com).

[mergify]

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