Ib404d29a662de7736b50a1d07380f651d332ad6b introduced a new sanity
check in RevisionStore::countRevisionsBetween to check whether
the ids are not null (otherwise, they're considered unsaved)
For MCR undo actions, diffs could be generated for unsaved revisions
(where one revision is a not yet pre-existing combination where
some slots remain unchanged, but some slot gets undone)
The existing if statement here was already trying to guard against
unsaved revisions, but was doing so in a way different from the
new checks in RevisionStore::countRevisionsBetween - even though this
was documented to only diff saved revisions, it wasn't checking
thoroughly enough, and an exception would crop up later.
Bug: T236320
Change-Id: If34766675f50b67d8b0788a6eab07d8d4e6fe183
7fe98fb56c