Fix edge case for #7075 where selection ends at beginning of last line of document

@redmunds - as I mentioned in email, this case is already fixed in the cmv4 version of the fix (so when I merge this over into cmv4 I'll just ignore this change, but keep the new unit test).

Note that there's an edge case of this edge case which is arguably still wrong :) - if the last line of the file is empty, then there's no way to delete it at the end of a block selection using Delete Line. I could imagine we add that as a special case - though it seems inconsistent, it might match what the user's intent usually is in this case. Let me know what you think.