net-snmp: add early return when HostName is unset

snmpd_access_HostName_add would write unintended output to the
config file if the HostName option is not specified. Add an early
return to prevent writing in that case.

This is a breaking change for configs where HostName is unset.
Previously, an empty HostName option would result in a malformed
directive with a trailing space and no guaranteed newline, the
exact form of which depended on the values of other options.
Now, no directive is written.

Signed-off-by: Eric McDonald <librick-openwrt@proton.me>

net/net-snmp/files/snmpd.init

@@ -143,6 +143,7 @@ snmpd_access_default_add() {
 snmpd_access_HostName_add() {
 	local cfg="$1"
 	config_get hostname "$cfg" HostName
+	[ -n "$hostname" ] || return 0
 	config_get mode "$cfg" Mode
 	[ -n "$mode" ] || return 0
 	config_get community "$cfg" CommunityName