Handle None value in nmcli tables

[diegobfernandez]

Fix for #428

There is no need to have another function as nmcli device wifi list returns a table that sparse_table_parse is already able to parse. The problem is _normalize_value is not handling None and failing when trying to call value.strip().

Short-circuit _normalize_value if value is None.

I did not added any tests because I couldn't setup the project, I've simply copied nmcli.py into $HOME/.local/share/jc/jcparsers and tested this change with that.

[kellyjonbrazil]

Hi and thanks for the contribution! I'm not seeing this fix in _normalize_value address #428:

% echo '* :AA\:AA\:AA\:AA\:AA\:AA:go_away:Infra:40:270 Mbit/s:25:▂___:WPA2' | jc --nmcli -pqd
Traceback (most recent call last):
  File "/Users/kbrazil/.local/bin/jc", line 33, in <module>
    sys.exit(load_entry_point('jc', 'console_scripts', 'jc')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kbrazil/git/jc/jc/cli.py", line 970, in main
    JcCli().run()
  File "/Users/kbrazil/git/jc/jc/cli.py", line 952, in run
    self._run()
  File "/Users/kbrazil/git/jc/jc/cli.py", line 916, in _run
    self.standard_parse_and_print()
  File "/Users/kbrazil/git/jc/jc/cli.py", line 808, in standard_parse_and_print
    self.create_normal_output()
  File "/Users/kbrazil/git/jc/jc/cli.py", line 760, in create_normal_output
    self.data_out = self.parser_module.parse(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kbrazil/git/jc/jc/parsers/nmcli.py", line 396, in parse
    if data.splitlines()[1].startswith('\t'):
       ~~~~~~~~~~~~~~~~~^^^
IndexError: list index out of range

Is that the same input you were testing with?