dm: core: Support writing a property to an empty node

At present this does not work with livetree. Fix it and add a test.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/ofnode.c

@@ -672,6 +672,9 @@ static int dm_test_ofnode_add_subnode(struct unit_test_state *uts)
 		malloc_disable_testing();
 	}
 
+	/* write to the empty node */
+	ut_assertok(ofnode_write_string(subnode, "example", "text"));
+
 	return 0;
 }
 DM_TEST(dm_test_ofnode_add_subnode,