device_tracker.ubus: Handle empty results (#7673)

If OpenWRT isn't running the DHCP server then some OpenWRT hardware,
such as TP-Link TL-WDR3600 v1, can't determine the host
corresponding to an associated wifi client. This change handles that
by returning None when the request has no data in the result.

homeassistant/components/device_tracker/ubus.py

@@ -144,7 +144,10 @@ def _req_json_rpc(url, session_id, rpcmethod, subsystem, method, **params):
         response = res.json()
 
         if rpcmethod == "call":
-            return response["result"][1]
+            try:
+                return response["result"][1]
+            except IndexError:
+                return
         else:
             return response["result"]