When flattening a query, make sure iTable attribute of TK_IF_NULL_ROW

operators (that result from a prior flattening of a LEFT JOIN) are updated
correctly.  Fix for ticket [cad1ab4cb7b0fc344].

FossilOrigin-Name: 92c178507df553e4f1110342c8f9b11b3ee37989e1d634fcaccabf657befa22f