Release file generation fails for series without components

If a suite has no components, apt-ftparchive will not create its directory in dists/. The publisher then crashes as it tries to create the Release file.

2010-11-13 23:01:09 ERROR Unexpected exception while doing release files
 -> http://launchpad.dev:58080/94/tdYwuAsARstF0VBbFGnLfI2UCv4.txt ([Errno 2] No such file or directory: u'/var/tmp/archive/ubuntu/dists/grumpy/Release')
2010-11-13 23:01:09 DEBUG3 abort
Traceback (most recent call last):
  File "./scripts/publish-distro.py", line 32, in <module>
    publishdistro.run_publisher(options, txn)
  File "/home/wgrant/launchpad/lp-branches/devel/lib/lp/soyuz/scripts/publishdistro.py", line 236, in run_publisher
    options.careful or options.careful_apt)
  File "/home/wgrant/launchpad/lp-branches/devel/lib/lp/soyuz/scripts/publishdistro.py", line 108, in try_and_commit
    func(*args)
  File "/home/wgrant/launchpad/lp-branches/devel/lib/lp/archivepublisher/publishing.py", line 350, in D_writeReleaseFiles
    self._writeSuite(distroseries, pocket)
  File "/home/wgrant/launchpad/lp-branches/devel/lib/lp/archivepublisher/publishing.py", line 530, in _writeSuite
    self._config.distsroot, suite, "Release"), "w")
IOError: [Errno 2] No such file or directory: u'/var/tmp/archive/ubuntu/dists/grumpy/Release'