Verify if offset exists

When i was trying to get the current Row from a ResultSet (`$query->getResultSet()->current();`) that can be empty, i'd got the follow error:
```
Notice: Undefined offset: 0 in vendor/everyman/neo4jphp/lib/Everyman/Neo4j/Query/ResultSet.php on line 54
```
So, i'm suggesting to verify the offset existence, then return null if it not exists.

Thanks in advance, and sorry for my english!