Handle `null` DTO projection conversion for primitive types

[meistermeier]

As raised here https://community.neo4j.com/t/sdn6-using-projection-persistence-with-primitive-fields/45177 SDN will fail with a NPE if the target field would be defined as a primitive.

[seabamirum]

This issue still occurs in 6.2.2. The NPE is now on line 77:

PreferredConstructor> constructor = targetEntity
.getPersistenceConstructor();