KeyError while accessing an empty Maildir

[Andrey Vlasovskikh <andrey.vlasovskikh@gmail.com>]

If a Maildir contains no message files, `hg ilist` raises the following
exception:

    KeyError: 'No message with key: None'

This can happen if one updates his repository to an early revision where there
was less issues than it is now. Mercurial doesn't delete empty directories, so
an issue from the future is represented a Maildir with no messages.

The problem is in handling the result of `_find_root_key`, which will be `None`
in that case.