Corrected a bug caused by the previous commit
Utilities: Perform string to array for GPDB 4.1

Changes:
    - String to array was being performed for versions less than
    GPDB 4.1. This has been changed to transform for versions less
    than GPDB 4.2.
    - Summary: ntile_array was transformed from string to array
    but the case where ntile_array is NULL was not handled earlier.