XWIKI-12920: Hiding WebHome in wiki link syntax

- Fixed NPE cause by the ExecutionContext/XWikiContext not being initialized by the time ConversionFilter is executed
-- Fixed by adding a null check in DAB.exists(documentReference) but other methods will fail if called in this timeslot.
-- Also using the XWikiContextProvider in DAB, but this does not help much, since that does nothing if ExecutionContext is not yet initialized, and it is not.
- Fixed test