netutils: dhcpc: Fix dhcpc.c

Summary:
- dhcpc now uses gethostname() but the hostname might be empty
- This commit fixes this issue

Impact:
- All use cases which use dhcpc

Testing:
- Tested with spresense:wifi

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

netutils/dhcpc/dhcpc.c

@@ -244,7 +244,7 @@ static int dhcpc_sendmsg(FAR struct dhcpc_state_s *pdhcpc,
 
   /* Get the current host name */
 
-  if (gethostname(hostname, sizeof(hostname)))
+  if (gethostname(hostname, sizeof(hostname)) || (0 == strlen(hostname)))
     {
       strncpy(hostname, CONFIG_NETUTILS_DHCPC_HOST_NAME, HOST_NAME_MAX);
     }