Fix the channels option

[ghost]

When python.notification_center.channels is set as "" in ~/.weechat/plugins.conf, notification_center.py doesn't work.
I investigated this issue and noticed if channel in channel_whitelist: of line 63 returns True although channel whitelist is not registered.
In Python, "".split(',') returns [''] and "" in [''] returns True....

To avoid the case, condition is set.

$ python
Python 3.8.1 (default, Feb  8 2020, 23:24:07)
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> "" in []
False
>>> "" in ['']
True
>>> whitelist = "".split(',')
>>> print (whitelist)
['']
>>> "" in whitelist
True

I confirmed the edited version of notification-center.py works in weechat.
If you would like to ask me more in detail, please let me know.