conf: always send response to parent waiting for devptfs_fd

conf: always send response to parent waiting for devptfs_fd

When no devpts devices are requested we used to return early but did not send a
response to the parent. This is a problem because the parent will be waiting
for a devpts fd to be sent. Make sure to always send a response.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>