None isn't a valid value of keybindings config anymore

[Felixoid]

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Describe the bug
None value in [keybindings] settings breaks the ability to set the keys.

To Reproduce

1. Set cycle_next = "" - and launch terminator, try to change the key binding for anything. It works as expected of shows a dialogue X already in use to trigger Y
2. Set cycle_next = None, now it's broken. I have config from an ancient version 2 years ago, back there it was a valid value

Expected behavior
I'd like to have the config backward compatible.

Desktop (please complete the following information):

• OS or Linux Distribution: Arch Linux
• Version terminator 2.1.1-2
• Display Technology: X11

Additional context

# logs for "" value
> terminator -u -g /tmp/term.config 

** (terminator:17429): WARNING **: 11:53:19.939: Binding '<Control><Alt>a' failed!
Unable to bind hide_window key, another instance/window has it.
<window.Window object at 0x7fe96198c540 (terminatorlib+window+Window at 0x55aeffa66280)> is not in registered window list
# logs for None value
> terminator -u -g /tmp/term.config   

** (terminator:17261): WARNING **: 11:52:54.237: Binding '<Control><Alt>a' failed!
Unable to bind hide_window key, another instance/window has it.
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/terminatorlib/prefseditor.py", line 1719, in on_cellrenderer_accel_edited
    parsed_conf_accel = Gtk.accelerator_parse(conf_accel)
TypeError: Argument 0 does not allow None as a value

[Vulcalien]

Hi! Thanks for bringing this up.
I could reproduce the bug, it shouldn't be hard to fix.