The touch trigger should update the timestamp column when it is currently NULL

The `column_name <> CURRENT_TIMESTAMP` behaves unexpectedly when `column_name` is NULL so if your timestamp columns start out as null then the timestamp is never updated by the trigger.

On postgres 9.2.4:

    spgt_test=# select NULL <> CURRENT_TIMESTAMP as result;
    result 
    --------
 
    (1 row)