Bugfix for 53290, fast unique index creation fails on duplicate null values

    Summary:
    Bug in the fast index creation code incorrectly considers null
    values to be duplicates during block merging.  Innodb policy is that
    multiple null values are allowed in a unique index.  Null duplicates
    were correctly ignored while sorting individual blocks and with slow
    index creation.

    Test Plan:
    mtr, including new test, load dbs using deferred index creation

    DiffCamp Revision: 110840
    Reviewed By: mcallaghan
    CC: mcallaghan, mysql-devel@lists
    Revert Plan:
    OK