test added

added test to reproduce error
bug fixed

added null cases to AkCollator.compare
added index scan to test so it chooses a plan with Distinct_Partial
merge JerettS: Distinct null string bug

Merge pull request #1408 from JerettS/sql-layer/DistinctNullStringBug
https://github.com/FoundationDB/sql-layer/pull/1408