Stack trace when package has no candidate in debug mode

This is what I get when running unattended-upgrades --debug:

Initial blacklisted packages:
Starting unattended upgrades script
...
Traceback (most recent call last):
  File "/usr/bin/unattended-upgrades", line 491, in <module>
    main()
  File "/usr/bin/unattended-upgrades", line 292, in main
    logging.debug("Checking: %s (%s)" % (pkg.name, map(str,
pkg.candidate.origins)))
AttributeError: 'NoneType' object has no attribute 'origins'

The problem is that the unattended-upgrades script doesn't check the value of pkg.candidate before calling the origins attribute on it. As can be seen in the apt.package.Package class, the candidate attribute might be None:

    def candidate(self):
        """Return the candidate version of the package.

        This property is writeable to allow you to set the candidate version
        of the package. Just assign a Version() object, and it will be set as
        the candidate version.
        """
        cand = self._pcache._depcache.get_candidate_ver(self._pkg)
        if cand is not None:
            return Version(self, cand)

Therefore, the unattended-upgrades script should expect this eventually. This can easily be done by using getattr and returning an empty list there are no candidates.