Can't unset the exit key

[lenormf]

Describe the bug
I don't want the program to quit upon hitting any one single key. I can't set exit_key to None or 0.

To Reproduce
Steps to reproduce the behavior:

1. Pass exit_key=None to the PyCUI constructor
2. Start the UI

Errors:

Traceback (most recent call last):
  File "/home/fle/misc/4dE/./4dE.py", line 65, in <module>
    sys.exit(main(sys.argv))
  File "/home/fle/misc/4dE/./4dE.py", line 51, in main
    root = UserInterface(2, 2)
  File "/home/fle/misc/4dE/./4dE.py", line 12, in __init__
    super().__init__(rows, columns, exit_key=None)
  File "/home/fle/misc/4dE/.env/lib/python3.9/site-packages/py_cui/__init__.py", line 141, in __init__
    exit_key_char = py_cui.keys.get_char_from_ascii(exit_key)
  File "/home/fle/misc/4dE/.env/lib/python3.9/site-packages/py_cui/keys.py", line 44, in get_char_from_ascii
    return chr(key_num)
TypeError: an integer is required (got type NoneType)

Upon passing exit_key=0, the program doesn't start (no output even in debug mode).

Expected behavior
The program doesn't close upon hitting any keys.

[lenormf]

My workaround for now is to pass exit_code=1 to the PyCUI constructor.

[PabloLec]

This could be implemented easily.

By returning None if key is None in PyCUI.get_char_from_ascii().
The only other thing to consider would be:

self._init_status_bar_text  = f'Press - {exit_key_char} - to exit. Arrow Keys to move ' \
                                       'between widgets. Enter to enter focus ' \
                                       'mode.'

A simple if-statement would be acceptable I think.

[jwlodek]

Also would need to make sure to somehow handle the use of the exit key in the main draw loop if that is None