Don't Fail On Non-Mappable Entities in Children

[zicklag]

Objective

Fixes #6790

Solution

• Don't fail when mapping entities that aren't found in the EntityMap.

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I don't think this requires a changelog entry, but let me know if I'm wrong.

Changelog

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Migration Guide

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[james7132]

This potentially makes an invalid hierarchy. This probably will panic when working with the propagation systems.

[zicklag]

Looks like we need another solution then.

I'll close this PR, so discussion can be continued in the issue, and we can open a new PR if we come up with a different solution.