sources/docker.py: Fix get_ref() to work when digest is not set

Currently, get_ref() does not work when self.digest is not set as it assumes that it is a string, which is not the case. This means that tracking a docker source which does not has any ref set will fail with a no-so-friendly message saying AttributeError: 'NoneType' object has no attribute 'startswith'.

Fix it so that get_ref() returns None when the ref is None.