Solves issue with skipped relationships on dense nodes

Assume a dense node had relationships only in one direction, say
 OUTGOING. Before this fix, if the node was not in cache and as
 the first operation on it all INCOMING relationships were retrieved,
 (which would return an empty iterator), then subsequent calls for
 OUGOING relationships would also return empty.
 This was caused by improperly assuming that an initialized but empty
 relationship array meant no relationships were present. This may be
 true for non dense nodes, but dense nodes need to be initialized
 essentially by type and direction.
This commit fixes the above issue and provides a test for it.
Merge pull request #2651 from digitalstain/2.1-maint

Solves issue with skipped relationships on dense nodes