Use Empty String for Cleared Accels

When a key binding is cleared its value is now set to "" (an empty
string) instead of `None` as before.

This change is introduced because `Gtk.accelerator_parse` does not
allow `None` to be used as a value, which in turn breaks
`on_cellrenderer_accel_edited` function in
`terminatorlib/prefseditor.py` by raising a `TypeError` every time a
new key binding is introduced. Note that this only happens if at
least one key binding has been cleared first.

terminatorlib/prefseditor.py

@@ -1745,7 +1745,7 @@ class PrefsEditor:
         liststore.set(celliter, 2, 0, 3, 0)
 
         binding = liststore.get_value(liststore.get_iter(path), 0)
-        self.config['keybindings'][binding] = None
+        self.config['keybindings'][binding] = ""
         self.config.save()
 
     def on_open_manual(self,  widget):