Correctly invalidate a column cache line when it is overwritten with NULL.

Fix for ticket [123c9ba32130a6c9d432].

FossilOrigin-Name: 65754dc3b830ef9482655d2e93c2cd4acaa7a8715a58d8309955d04378541f89

src/wherecode.c

@@ -1690,6 +1690,7 @@ Bitmask sqlite3WhereCodeOneLoopStart(
       }
     }else if( bStopAtNull ){
       sqlite3VdbeAddOp2(v, OP_Null, 0, regBase+nEq);
+      sqlite3ExprCacheRemove(pParse, regBase+nEq, 1);
       endEq = 0;
       nConstraint++;
     }