livefs builds fail when they produce an empty artifact

Some livefs builds are currently failing like this:

2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] Failed to grab http://lgw01-amd64-035.vbuilder.lgw01.scalingstack:8221/filecache/da39a3ee5e6b4b0d3255bfef95601890afd80709: 'NoneType' object has no attribute 'name'
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] Traceback (most recent call last):
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 568, in _startRunCallbacks
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] self._runCallbacks()
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] current.result = callback(current.result, *args, **kw)
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 460, in callback
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] self._startRunCallbacks(result)
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 568, in _startRunCallbacks
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] self._runCallbacks()
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] --- <exception caught here> ---
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] current.result = callback(current.result, *args, **kw)
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/lib/lp/buildmaster/interactor.py", line 257, in got_response
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] response.deliverBody(FileWritingProtocol(finished, file_to_write))
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/web/_newclient.py", line 1050, in dispatcher
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] return func(*args, **kwargs)
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/env/local/lib/python2.7/site-packages/twisted/web/_newclient.py", line 1219, in _deliverBody_DEFERRED_CLOSE
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] protocol.connectionLost(self._reason)
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] File "/srv/launchpad.net/codelines/soyuz-production-rev-19034/lib/lp/buildmaster/interactor.py", line 90, in connectionLost
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] os.rename(self.file.name, self.filename)
2019-08-28 08:47:03+0000 [HTTP11ClientProtocol,client] exceptions.AttributeError: 'NoneType' object has no attribute 'name'

The file in question is livecd.magic-proxy.log. This happens when a build artifact is empty. While this may or may not be a bug in the livefs build in question, Launchpad shouldn't fail the build obscurely in the face of this.