Do not send disk usage alarm when the data is null value

When the Agent client can't get disk information, it fills all data to zero.
The Agent server should ignore the null value data.

Author: Park Juhyung

server/src/agent/agent.rs

@@ -344,7 +344,7 @@ impl Agent {
 
                 const ONE_GB: i64 = 1_000_000_000;
                 if !disk_usage_alert_sent {
-                    if disk_usage.available < ONE_GB {
+                    if disk_usage.total != 0 && disk_usage.available < ONE_GB {
                         self.noti.warn(
                             &network_id,
                             &format!("{} has only {} MB free space.", node_name, disk_usage.available / 1_000_000),