Hashed storage migration: robustness if wiki repo is missing
A fix for an old version that will help migrate to the newer ones!
As seen in #742 (closed), in Heptapod 0.23, if a Project lacks its wiki repository, then the hashed storage migration fails. It should not.
I don't have on mind whether the wiki repo should exist if the feature is disabled in the Project settings, this has to be checked. If not, that is also an easy recommendation to administrators of the affected instance: just disable it on the offending Projects.