[source] dnsmasq: fix dhcp_option usage warning

[LEDE Commits]

dedeckeh pushed a commit to source.git, branch master:
https://git.lede-project.org/cd5cd7c859660a52a09d6744a4343e3078a8c883

commit cd5cd7c859660a52a09d6744a4343e3078a8c883
Author: Hans Dedecker <dedeckeh at gmail.com>
AuthorDate: Thu May 4 22:33:37 2017 +0200

    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 at gmail.com>
---
 package/network/services/dnsmasq/files/dnsmasq.init | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/package/network/services/dnsmasq/files/dnsmasq.init b/package/network/services/dnsmasq/files/dnsmasq.init
index 4291df2..e6ba732 100644
--- a/package/network/services/dnsmasq/files/dnsmasq.init
+++ b/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"