Update #8007 wrongly assumes that all enabled entity types have a revision table

While updating to Multiversion 8.x-1.0-beta1, I discovered something icky.

Update #8007 tries to read data from the revision tables for all enabled entity types (as compiled by update #8005), but it is wrongly assuming that all of those entity types actually have a revision table. If it encounters one that does, it tries to select rows from a null table and a PDOException is thrown. I'm don't know what the ultimate result of this is, but it seems reasonable to assume that it can lead to data corruption. Thus, I think this bug is critical.

I'm not sure what the proper solution would be, but a quick-fix is for the #8007 update hook to at least sanity-check that $table_revision is not empty. Patch coming shortly.