DBS Placeless Documents return null acp

TestNuxeoDriveManager#testGetSyncRootsWithPlacelessDocument shows that DBSSession#getMergedACP returns null on placeless documents, which leads to an NPE. This null ACP is the one contained by the DBSDocument representing the placeless document.
In contrary SQLSession#getMergedACP returns the ACP of the SQLDocumentLive representing the placeless document.

DBSSession#getMergedACP and SQLSession#getMergedACP contain strictly the same code though. (Maybe they could be refactored ?)

The difference is one manipulates an SQLDocumentLive which has an ACP and the other manipulates a DBSDocument with a null ACP