Fix crash caused by deleting non-existing relationship in DETACH DELETE clause

[DavIvek]

This PR is about preventing a crash caused by deleting a non-existing relationship.

ATM query:
CREATE (n0)<-[r0:T3]-(n1) DETACH DELETE n0 DETACH DELETE r0 RETURN n1;
will crash the database.

The problem is in the second DETACH DELETE clause in which we are trying to detach relationship r0 from its edges, since edge n0 is deleted by the previous DETACH DELETE clause, that will crash a database.

[master < Task] PR

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closes #1329