Fixes unmark clearing behavior on empty input

It looks like when `unmark` is sent to i3 and it contains no identifier,
its default behavior is to clear all marks. This commit makes the
`unmark` script test if the mark is empty and do nothing if empty,
instead of deleting all marks.
Removes unnecessary echo to dzen

Leftover tidbit from testing. Not needed.
Fixes if-statement. Working unmark.
Merge pull request #5 from nathantypanski/master

Fixes clearing all marks when no target is chosen