Fix NopReceiver.ready() to properly terminate when receiver is closed.#427

ela-kotulska-frequenz merged 2 commits intofrequenz-floss:v1.x.xfrequenz-floss/frequenz-channels-python:v1.x.xfrom

ela-kotulska-frequenz:fix_nop_receiverela-kotulska-frequenz/frequenz-channels-python:fix_nop_receiverCopy head branch name to clipboard