Battery_level: Prevent logic issues when zero

[tobes]

When a value of zero was obtained we could end up with a value of None. This fixes the issue

TypeError: unsupported operand type(s) for /: 'int' and 'NoneType'
  File "/home/toby/dev/py3status/py3status/module.py", line 584, in run
    response = method()
  File "/home/toby/dev/py3status/py3status/modules/battery_level.py", line 174, in battery_level
    self._refresh_battery_info()
  File "/home/toby/dev/py3status/py3status/modules/battery_level.py", line 316, in _refresh_battery_info
    battery_list = self._extract_battery_information_from_sys()
  File "/home/toby/dev/py3status/py3status/modules/battery_level.py", line 294, in _extract_battery_information_from_sys
    time_in_secs = (remaining_energy / present_rate * 3600)

[ultrabug]

Thanks mate !