First commit for DM8 - there were some FPs (#5894)

lib/core/dicts.py

@@ -269,7 +269,7 @@ FROM_DUMMY_TABLE = {
 HEURISTIC_NULL_EVAL = {
     DBMS.ACCESS: "CVAR(NULL)",
     DBMS.MAXDB: "ALPHA(NULL)",
-    DBMS.MSSQL: "DIFFERENCE(NULL,NULL)",
+    DBMS.MSSQL: "IIF(1=1,DIFFERENCE(NULL,NULL),0)",
     DBMS.MYSQL: "QUARTER(NULL XOR NULL)",
     DBMS.ORACLE: "INSTR2(NULL,NULL)",
     DBMS.PGSQL: "QUOTE_IDENT(NULL)",