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/vim9class.c b/src/vim9class.c
index c8cea3c..74a9bf3 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -269,6 +269,7 @@
 		ga_concat(&fga, (char_u *)"this.");
 		objmember_T *m = cl->class_obj_members + i;
 		ga_concat(&fga, (char_u *)m->om_name);
+		ga_concat(&fga, (char_u *)" = v:none");
 	    }
 	    ga_concat(&fga, (char_u *)")\nenddef\n");
 	    ga_append(&fga, NUL);