Do not allow nullable new_id

The usefulness of this is limited, and `libwayland-client` doesn't
provide a way to pass a null `new_id` since the id is generated by the
library and given to the caller as the return value.

Signed-off-by: Ian Douglas Scott <idscott@system76.com>

tests/connection-test.c

@@ -245,9 +245,6 @@ TEST(connection_marshal)
 	marshal(&data, "n", 12, &object);
 	assert(data.buffer[2] == object.id);
 
-	marshal(&data, "?n", 12, NULL);
-	assert(data.buffer[2] == 0);
-
 	array.data = (void *) text;
 	array.size = sizeof text;
 	marshal(&data, "a", 20, &array);