XWIKI-9729: Stracktrace during the DW when upgrading a workspace from 4.5.4 to 5.3M2.

* Fixed.
XWIKI-9729: Stracktrace during the DW when upgrading a workspace from 4.5.4 to 5.3M2.

* Cleaner fix.
Merge pull request #213 from gdelhumeau/XWIKI-9729

XWIKI-9729: Stracktrace during the DW when upgrading a workspace from 4.5.4 to 5.3M2.