Fix default gui_client assignment

There was a typo and it said `ni` rather than `nil`. Luckily (or unluckily, it
depends on how you see it), the variable `ni` isn't defined anywhere so it
default to... `nil`.

Not worth making another release with this change only as nothing actually
breaks, so it will end up in the next release.

power_widget.lua

@@ -171,7 +171,7 @@ local function init(widget)
   )
 
   widget.tooltip = awful.tooltip({ objects = { widget },})
-  widget.gui_client = ni
+  widget.gui_client = nil
   widget.critical_percentage = 5
 
   widget.warning_config = {