Add a test for the problem.
Assemble null index key as literal null.
Add a test demonstrating why HKeyRow_Default still needs to output even with a NULL component.
merge MMcM: Assemble IS NULL condition for HKeyRow_Default

Merge pull request #1204 from MMcM/sql-layer/null-hkey-row
https://github.com/FoundationDB/sql-layer/pull/1204