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/parser/features/function_many_parameters.out

@@ -1,2 +1,2 @@
 GDTEST_OK
-123456789101112131415161718192212223242526272829303132333435363738394041424344454647falsetruenull
+123456789101112131415161718192212223242526272829303132333435363738394041424344454647falsetrue<null>

modules/gdscript/tests/scripts/parser/features/str_preserves_case.out

@@ -1,4 +1,4 @@
 GDTEST_OK
-null
+<null>
 true
 false