This is a draft solution for #8723. This PR moves nullable logic on JoinColumn from ClassMetadataInfo to drivers and implements reflection fallback for Annotation and Attribute drivers.

Logic here is to use property nullability if and only if there are JoinColumn annotation/attributes that doesn't have nullable set, thus limiting possible BC to situation when JoinColumn is set without nullable and the property is not nullable.

This logic can be extended to other drivers if desired. Also I didn't add any tests (yet!) to get the approach reviewed first.