Prevent errors in prompts if no info available.

Define empty functions instead of none at all if we can't figure out the
platform.

plugins/battery/battery.plugin.zsh

@@ -83,4 +83,14 @@ elif [[ $(uname) == "Linux"  ]] ; then
       echo "∞"
     fi
   }
+else
+	# Empty functions so we don't cause errors in prompts
+	function battery_pct_remaining() {
+	}
+
+	function battery_time_remaining() {
+	}
+
+	function battery_pct_prompt() {
+	}
 fi