add structure assignment optional field test

tests/unit/issues/Issue2646.hx

@@ -13,5 +13,16 @@ class Issue2646 extends unit.Test
 		var d = { c : c };
 		t(d.c.b.a == null);
 		f(d.c.b.a != null);
+		
+		var a = {};
+		var b:{?a:Bool} = a;
+		t(b.a == null);
+		f(b.a != null);
+		var c = { b: b };
+		t(c.b.a == null);
+		f(c.b.a != null);
+		var d = { c : c };
+		t(d.c.b.a == null);
+		f(d.c.b.a != null);
 	}
 }