[LiveComponent] Fix (de)hydration of composite and/or foreign ID entities

[MatTheCat]

Q	A
Bug fix?	yes
New feature?	no
Issues	N/A
License	MIT

This PR fixes two issues (tests included) with the DoctrineEntityHydrationExtension:

• Entities with composite IDs couldn’t be hydrated because in this case the object manager needs the properties as keys. Throwing if the 0 key does not exist prevents this to work, so this PR removes this check.
• Dehydrated entities whose ID is a foreign key would bear the corresponding entity (or proxy), preventing their future correct hydration. This PR dehydrates ID values as well if needed.

[kbond]

This is great, thanks Mathieu!