Fix iterating over relationship data generator when generator is empty #252

[lindyhopchris]

As described in the issue, if relationship data was an empty generator, an error would be triggered as the generator was iterated over twice when including related resources.

This is because the cache in RelationshipDataIsCollection was not being set to an empty array.

This PR adds two tests: one for a non-empty generator, and another for an empty generator. The empty generator test was failing before the single line fix was put into RelationshipDataIsCollection.

Closes #252