AtlasEntity::addRelationship fails silently due to a wrong check

[gse89]

Describe the bug
The AtlasEntity class method addRelationship checks if the relationship attributes are uninitialised by wrongly testing if the customAttributes attribute (instead of the relationshipAttributes one) was initialised.

To Reproduce
Steps to reproduce the behavior:

1. Create an AtlasEntity
2. Set a customAttribute
3. Try to add a relationship by calling AtlasEntity::addRelationship
4. Verify that AtlasEntity.relationshipAttributes is still empty

Expected behavior
The AtlasEntity object should have the added relationship.

Desktop (please complete the following information):

• OS: Mac
• PyApacheAtlas Version (python -m pyapacheatlas --version): 0.15.0