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

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

modules/mono/glue/runtime_interop.cpp

@@ -1319,7 +1319,7 @@ void godotsharp_object_to_string(Object *p_ptr, godot_string *r_str) {
 #endif
 	// Can't call 'Object::to_string()' here, as that can end up calling 'ToString' again resulting in an endless circular loop.
 	memnew_placement(r_str,
-			String("[" + p_ptr->get_class() + ":" + itos(p_ptr->get_instance_id()) + "]"));
+			String("<" + p_ptr->get_class() + "#" + itos(p_ptr->get_instance_id()) + ">"));
 }
 
 #ifdef __cplusplus