GDScript: Fix cast producing null

modules/gdscript/tests/scripts/analyzer/features/cast_non_null.gd

@@ -0,0 +1,5 @@
+# https://github.com/godotengine/godot/issues/69504#issuecomment-1345725988
+
+func test():
+	print("cast to Variant == null: ", 1 as Variant == null)
+	print("cast to Object == null: ", self as Object == null)

modules/gdscript/tests/scripts/analyzer/features/cast_non_null.out

@@ -0,0 +1,3 @@
+GDTEST_OK
+cast to Variant == null: false
+cast to Object == null: false