If GetTor deals with an autoresponder, it returns an empty request,
which is then passed to parse_callback, which raises a KeyError because
of request["command"]:

2020-05-29 19:04:43+0000 [email parser] Error while parsing email content: [Failure instance: Traceback: <class 'KeyError'>: 'command'
	/usr/lib/python3/dist-packages/twisted/internet/defer.py:311:addCallbacks
	/usr/lib/python3/dist-packages/twisted/internet/defer.py:654:_runCallbacks
	/usr/lib/python3/dist-packages/twisted/internet/defer.py:1613:unwindGenerator
	/usr/lib/python3/dist-packages/twisted/internet/defer.py:1529:_cancellableInlineCallbacks
	--- <exception caught here> ---
	/usr/lib/python3/dist-packages/twisted/internet/defer.py:1418:_inlineCallbacks
	/srv/gettor.torproject.org/home/gettor/gettor/parse/email.py:256:parse_callback
	].

This is a fixup of: <https://bugs.torproject.org/34286>