Delete items that are edited to the empty string.

When using the edit feature of browse-kill-ring, if an item is edited to the empty string then it is deleted from the `kill-ring' and the display is updated.

NOTE: This delete feature uses the same delete method as is used in `browse-kill-ring-delete`, however, both this delete, and the one in `browse-kill-ring-delete` are a little bit broken, they both actually delete all matching entries from the kill ring rather than just the current entry.  I think for now this is acceptable, this is a long standing bug, I do hope to fix it, but the issue is separate to this one (I believe).