DERBY-6374 Bulk insert of data with nullable UNIQUE constraint fails to detect duplicates

Backported to 10.10, had to do major surgery since the original fix
was part of a DERBY-532 patch.



git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.10@1579737 13f79535-47bb-0310-9956-ffa450edef68