Loosen the usb_reset condition so that usb_reset has to be truthy.

With the homeassistant add-on, it's currently impossible to turn this
feature off.

  - The default value of usb_reset is "005:002"
  - Setting usb_reset explicitly to null via `usb_reset:` ends up
getting overridden with the default value
  - Setting usb_reset to the empty string doesn't disable the feature,
until now

rtlamr2mqtt.py

@@ -392,7 +392,7 @@ def sliding_mean(series):
 
 # Main loop
 while True:
-    if usb_reset is not None:
+    if usb_reset:
         reset_usb_device(usb_reset)
 
     mqtt_sender.publish(topic=availability_topic, payload='online', retain=True)