Fix a breakage in move detection when moving from the end of a file.
The move detection logic tries a look-ahead for a blank line on the

source range, which assumed there would be a line following that range.

That's not true when moving the very last lines in a file, though.
This change only does the look-ahead if there is a line to look ahead

at.

My broken change that moved code from the very end of the file no longer

breaks.