- Fix PeriodicScheduler when a build is triggered with no changes

when the PeriodicScheduler trigger a build with no changes, we have this traceback: 
``` 
	Traceback (most recent call last):
	Failure: twisted.internet.defer.FirstError: FirstError[#0, [Failure instance: Traceback: <type 'exceptions.AssertionError'>: project cannot be None
	/home/xavierd/Projects/myGitHub/buildbot-work/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py:1099:_inlineCallbacks
	/home/xavierd/Projects/myGitHub/buildbot-work/src/master/buildbot/db/buildsets.py:60:addBuildset
	/home/xavierd/Projects/myGitHub/buildbot-work/src/master/buildbot/db/buildsets.py:58:toSsid
	/home/xavierd/Projects/myGitHub/buildbot-work/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py:1237:unwindGenerator
	--- <exception caught here> ---
	/home/xavierd/Projects/myGitHub/buildbot-work/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py:1099:_inlineCallbacks
	/home/xavierd/Projects/myGitHub/buildbot-work/src/master/buildbot/db/sourcestamps.py:45:findSourceStampId
	]]
```