Correct nullability annotation for ReferenceEntry

[roji]

Unfortunately, the situation here is complicated, since ReferenceEntry APIs sometimes returns nullable for TProperty, sometimes not:

• ReferenceEntry.CurrentValue can return nulls if the reference navigation property was nullable.
• ReferenceEntry.TargetEntry returns a nullable EntityEntry, so the TProperty it wraps is never null. Query also return IQueryable<TProperty> where TProperty should never be null.

I don't think there's a perfect solution, here are some options with what they mean for the return values of ReferenceEntry.TargetEntry and CurrentValue:

	TargetEntry for nullable property	TargetEntry for non-nullable property	CurrentValue for nullable property	CurrentValue for non-nullable property
1. Only change EntityEntry.Reference to accept nullable property (no change to ReferenceEntry)	EntityEntry<BlogDetails>?, good	EntityEntry<BlogDetails>?, good	BlogDetails, really bad	BlogDetails, good
2. (1) + change ReferenceEntry.CurrentValue to return TProperty?	EntityEntry<BlogDetails>?, good	EntityEntry<BlogDetails>?, good	BlogDetails?, good	BlogDetails?, bad
3. (1) + change ReferenceEntry to accept class? constraint	EntityEntry<BlogDetails?>?, bad	EntityEntry<BlogDetails>?, good	BlogDetails?, good	BlogDetails, good

• It's always better to return non-nullable as nullable (only forcing an extra check) rather than to return nullable as non-nullable (causing unexpected NRE) - so option (1) is the worst.
• Option (3) also requires changing EntityEntry's constraint to class?, since ReferenceEntry.TargetEntry returns that; this means EntityEntry.Entry would return nullable when reached through a nullable reference navigation - not ideal. It would also make ReferenceEntry.Query return IQueryable<TProperty?>, which isn't good.
• So option (2) seems the least bad - the only downside is that users need an extra annoying null check when accessing ReferenceEntry.CurrentValue. This is what this PR implements.

Fixes #24659

[AndriySvyryd]

Consider merging this to rc1 since it impacts public API