dnsmasq: fix dhcp_option usage warning

Don't display unnecessary dhcp_option usage warning in case
dhcp_option is empty

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

package/network/services/dnsmasq/files/dnsmasq.init

@@ -560,9 +560,10 @@ dhcp_option_add() {
 	if [ -n "$list_len" ]; then
 		config_list_foreach "$cfg" dhcp_option dhcp_option_append "$networkid" "$force"
 	else
-		echo "Warning: the 'option dhcp_option' syntax is deprecated, use 'list dhcp_option'" >&2
 		config_get dhcp_option "$cfg" dhcp_option
 
+		[ -n "$dhcp_option" ] && echo "Warning: the 'option dhcp_option' syntax is deprecated, use 'list dhcp_option'" >&2
+
 		local option
 		for option in $dhcp_option; do
 			dhcp_option_append "$option" "$networkid" "$force"