Fixes clearing all marks when no target is chosen

By default, i3 will clear all marks if `unmark` is sent without a target, which deletes all marks when the user hits `ESC` to exit `dmenu`. This adds an `if` block that tests whether the user entered nothing before trying to `unmark`. It now only messages i3 if the user actually selected a target, so all marks never get cleared at once.

It also adds a prompt to `unmark`, matching that of `goto` and `mark`.