fix error on GATEWAY_MODEL_ZIG3 when no zigbee devices connected (#1065)

* fix error on GATEWAY_MODEL_ZIG3 when no zigbee devices connected

* clean up

* black formatting

miio/gateway/gateway.py

@@ -170,6 +170,12 @@ def discover_devices(self):
             # self.send("get_device_list") does work for the GATEWAY_MODEL_ZIG3 but gives slightly diffrent return values
             devices_raw = self.send("get_device_list")
 
+            if type(devices_raw) != list:
+                _LOGGER.debug(
+                    "Gateway response to 'get_device_list' not a list type, no zigbee devices connected."
+                )
+                return self._devices
+
             for device in devices_raw:
                 # Match 'model' to get the model_info
                 model_info = self.match_zigbee_model(device["model"], device["did"])