Fixed executeBase filter builder to support nulls
Fixed executeBase filter builder to support nulls

$db->quote() returns 'null' if value is_null(), but the comparer doesn't allow for this; this commit changes comparer to "IS" if value is_null()
MichaelMaar/concrete5,MichaelMaar/concrete5,matt9mg/concrete5,TimDix/concrete5,matt9mg/concrete5,MichaelMaar/concrete5,avdevs/concrete5,TimDix/concrete5,avdevs/concrete5,TimDix/concrete5,matt9mg/concrete5,avdevs/concrete5