Fix deepCopy with null join conditions

At one point the yaml test in this PR caused a NullPointerException in JoinNode#deepCopy. However, https://github.com/FoundationDB/sql-layer/pull/1470 inadvertently fixed this test by bypassing deepCopy. Currently there doesn't appear to be a query that would cause a call to JoinNode.deepCopy, so a unit test was added. 

Also, some code that can never be hit was removed.
Doesn't this get hit if you have multiple unions (e.g. `SELECT x FROM T UNION SELECT x FROM T2 UNION SELECT x FROM T3`)