Remove empty checkboxes from rofi

This is a change in `rofi` 1.7.4 and looks ugly in `rofimoji`. To
circumvent this, we remove the empty checkboxes again and get something
similar to the previous look.

Thanks to @alpheratz0 for finding the problematic commit!

Issue: #127

src/picker/selector.py

@@ -63,6 +63,8 @@ def show_character_selection(
             '-markup-rows',
             '-i',
             '-multi-select',
+            '-ballot-unselected-str',
+            '',
             '-p',
             prompt,
             '-kb-custom-11',