Launchpad API can return a null preview_diff and prevents triggering

[Francis Ginther]

Looks like launchpad can return 'None' for the preview diff. This results in no triggering.

The MP in question was a resubmit of one developers branch over another developers:
https://code.launchpad.net/~fginther/ubuntu-docviewer-app/emulator-integration/+merge/189486

[log]
2013-10-07 19:17:33,035 DEBUG Triggering lp:ubuntu-docviewer-app
2013-10-07 19:17:33,035 DEBUG /var/lib/jenkins/jenkins-launchpad-plugin/launchpadTrigger.py --lock-name=phablet-ubuntu-touch-coreapps.cfg --branch=lp:ubuntu-docviewer-app --job=ubuntu-docviewer-app-autolanding --autoland --fasttrack
DEBUG: Going to acquire launchpad lock "launchpad-trigger-lock-phablet-ubuntu-touch-coreapps.cfg" for launchpadTrigger
DEBUG: Lock acquired for launchpadTrigger
DEBUG: Using "/var/lib/jenkins/.launchpadlib-phablet-ubuntu-touch-coreapps.cfg" as launchpadlib cache directory.
DEBUG: jenkins job parameter [use_description_for_commit]: False
DEBUG: Job is configured to fallback to description: False
DEBUG: Going to trigger merge: https://api.launchpad.net/1.0/~fginther/ubuntu-docviewer-app/emulator-integration
ERROR: preview_diff of this merge proposal is None
DEBUG: Skipping this merge proposal
2013-10-07 19:17:38,228 ERROR Command ['/var/lib/jenkins/jenkins-launchpad-plugin/launchpadTrigger.py', '--lock-name=phablet-ubuntu-touch-coreapps.cfg', '--branch=lp:ubuntu-docviewer-app', '--job=ubuntu-docviewer-app-autolanding', '--autoland', '--fasttrack'] returned non-zero exit status 1