Omit NOT NULL checks on unchanging columns in an UPDATE.

FossilOrigin-Name: 6a3aaedfb41735996470abbae6d3cd1be1f508b3

src/update.c

@@ -572,7 +572,8 @@ void sqlite3Update(
     /* Do constraint checks. */
     assert( regOldRowid>0 );
     sqlite3GenerateConstraintChecks(pParse, pTab, aRegIdx, iDataCur, iIdxCur,
-        regNewRowid, regOldRowid, chngKey, onError, labelContinue, &bReplace);
+        regNewRowid, regOldRowid, chngKey, onError, labelContinue, &bReplace,
+        aXRef);
 
     /* Do FK constraint checks. */
     if( hasFK ){