dependency-scanning customisation fails when a project has no requirements

When a project has no requirements (e.g. only the standard library), the python/dependency-scanning component falls over:

$ python - <<EOF # collapsed multi-line command
running dist_info
creating /builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info
writing /builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/PKG-INFO
writing dependency_links to /builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/dependency_links.txt
writing entry points to /builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/entry_points.txt
writing top-level names to /builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/top_level.txt
writing manifest file '/builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/SOURCES.txt'
reading manifest file '/builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
adding license file 'LICENSE'
writing manifest file '/builds/duncanmmacleod/igwn-archive/igwn_archive.egg-info/SOURCES.txt'
creating '/builds/duncanmmacleod/igwn-archive/igwn_archive-0.1.1.dist-info'
Creating requirements.txt in /builds/duncanmmacleod/igwn-archive/
Traceback (most recent call last):
  File "<stdin>", line 27, in <module>
TypeError: 'NoneType' object is not iterable