p/cpu_: fix test for guest_nice emptyness

The simple "=" is prefered to the "==" one, as we don't need to test numeric
values.

plugins/system/cpu_

@@ -87,9 +87,7 @@ emit_values()
 {
 	while read key user nice system idle iowait irq softirq steal guest guest_nice
 	do
-		if [ "$guest_nice" == "" ]; then
-			guest_nice=0
-		fi
+		[ "$guest_nice" = "" ] && guest_nice=0
 		[ "$key" != "$1" ] && continue
 		cat <<EOF
 system.value $system