fix native string subclass construction without argument

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/BuiltinConstructors.java

@@ -2219,7 +2219,11 @@ static Object doNativeSubclass(VirtualFrame frame, Object cls, Object obj, @Supp
                         @Cached @SuppressWarnings("unused") IsSubtypeNode isSubtype,
                         @Shared @Cached PyObjectStrAsObjectNode strNode,
                         @Cached CExtNodes.StringSubtypeNew subtypeNew) {
-            return subtypeNew.call(cls, strNode.execute(frame, obj));
+            if (obj == PNone.NO_VALUE) {
+                return subtypeNew.call(cls, T_EMPTY_STRING);
+            } else {
+                return subtypeNew.call(cls, strNode.execute(frame, obj));
+            }
         }
 
         protected static boolean isSubtypeOfString(VirtualFrame frame, IsSubtypeNode isSubtypeNode, Object cls) {