Fix an issue with globbing for BAT* when it is not present. On Ubuntu 14...

Fix an issue with globbing for BAT* when it is not present. On Ubuntu 14.04, machines without a battery don't have any BAT* devices, and zsh throws an error instead of passing an empty glob. http://zshwiki.org/home/options/expn has documentation for the null_glob option.

Prior to this change, machines without a battery print

__promptline_battery:27: no matches found: /sys/class/power_supply/BAT*

before each prompt.