Machine files raise lots of errors with -Wnullable-to-nonnull-conversion

[ibsh]

With the introduction of proper nullability in Objective-C machine consumption headers (thank you!), there are multiple errors raised with the -Wnullable-to-nonnull-conversion flag (which my team has adopted for ease of adding Swift to our legacy Objective-C codebase). For example (and this is almost certainly not exhaustive):

1. All implementations of +entityInManagedObjectContext: are invalid, as the internal NSEntityDescription#entityForName:inManagedObjectContext: call has a nullable return
2. Implementations of CoreDataGeneratedAccessors are invalid for nullable collection properties

I guess for 1), it would be best to mark +entityInManagedObjectContext: as nullable.
Conditional code will be necessary for 2).

[rentzsch]

Thanks!