Fixed NullPointerException in EntityGraphMapper::mapEntityReference

[oxisto]

Seems to be an obvious fix, not sure if this is really enough.

Fixes #1132

[michael-simons]

Thanks for your contribution, @oxisto

I added a test for that scenario onto your branch.
In general, this is kinda broken (the scenario): It can only happen if you

• have a relationship entity class
• Purposely use a different type on the ends in the node than in the relationship class

While it now won't generate an NPE, you would not be able to store properties this way and I surely recommend fixing the types (use the same type in both @Relationship and @RelationshipEntity.