patch 9.0.1053: default constructor arguments are not optional

Problem:    Default constructor arguments are not optional.
Solution:   Use "= v:none" to make constructor arguments optional.
diff --git a/src/errors.h b/src/errors.h
index 13d947b..9c8d593 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3372,4 +3372,6 @@
 	INIT(= N_("E1326: Member not found on object \"%s\": %s"));
 EXTERN char e_object_required_found_str[]
 	INIT(= N_("E1327: Object required, found %s"));
+EXTERN char e_constructor_default_value_must_be_vnone_str[]
+	INIT(= N_("E1328: Constructor default value must be v:none: %s"));
 #endif