Fix battery_percentage check in Atomic theme

Check for the return value "no", which is returned when no battery check funcationality is available in the OS. This is now consistent with how the Brainy theme does this.

themes/atomic/atomic.theme.bash

@@ -164,8 +164,10 @@ ___atomic_prompt_clock() {
 }
 
 ___atomic_prompt_battery() {
-  chk=$(command_exists battery_percentage && echo yes || echo no)
-  [ "$chk" != "yes" ] || [ "${THEME_SHOW_BATTERY}" != "true" ] && return
+  ! command_exists battery_percentage ||
+  [ "${THEME_SHOW_BATTERY}" != "true" ] ||
+  [ "$(battery_percentage)" = "no" ] && return
+
   batp=$(battery_percentage)
   if [ "$batp" -eq 50 ] || [ "$batp" -gt 50 ]; then
     color=$bold_green