Fix handling of volatile expressions in COPY FROM ... WHERE

The checking for calls to volatile functions in the COPY FROM ... WHERE
expression was treating all WHERE clauses as if containing such calls.
While that does not produce incorrect results, this disables batching
which may result in significant performance regression.

Discussion: https://www.postgresql.org/message-id/flat/CALAY4q_DdpWDuB5-Zyi-oTtO2uSk8pmy+dupiRe3AvAc++1imA@mail.gmail.com

src/backend/commands/copy.c

@@ -2612,8 +2612,7 @@ CopyFrom(CopyState cstate)
 		 */
 		insertMethod = CIM_SINGLE;
 	}
-	else if (cstate->whereClause != NULL ||
-			 contain_volatile_functions(cstate->whereClause))
+	else if (contain_volatile_functions(cstate->whereClause))
 	{
 		/*
 		 * Can't support multi-inserts if there are any volatile funcation