MappingContext should not consider java.util.Optional an entity type

[christophstrobl]

While this has been fine for quite a while the tightened access restrictions as of Java16 cause trouble when trying to open the protected constructor via reflection.
Further more considering a nullable wrapper type, like Optional, a persistent entity is problematic, cause it is the actual type argument that should be taken into account.

Things to do:

• Introduce nullable wrapper check to TypeInformation.
• Return the component type of the wrapper for TypeInformation#getActualType.
• Use the nullable component type when adding persistent entities for a property.