fixed parser error when parsing order by with nulls order clause (#12)

* fixed parser error when parsing order by with nulls order clause

* pr remarks

Author: dannleed

grammars/SQLSelectParser.g4

@@ -188,7 +188,7 @@ olapOption:
 ;
 
 orderClause:
-    ORDER_SYMBOL BY_SYMBOL orderList
+    ORDER_SYMBOL BY_SYMBOL orderList nullsOrder? 
 ;
 
 direction:
@@ -822,6 +822,10 @@ orderList:
     orderExpression (COMMA_SYMBOL orderExpression)*
 ;
 
+nullsOrder:
+    NULLS_SYMBOL (FIRST_SYMBOL | LAST_SYMBOL)
+;
+
 orderExpression:
     expr direction?
 ;