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.