Improve null and object printing to avoid confusion with arrays

- Use different syntax for object printing to avoid confusion with arrays.
- Print null as `<null>` to avoid confusion with a string `"null"`.
- Display `<empty>` in editor resource pickers to avoid confusion
  with array-based properties.

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

@@ -1,5 +1,5 @@
 GDTEST_OK
-null
+<null>
 0
 1
 2