[device_tracker] Don't clear GPS coordinates if no GPS seen (#4848)

homeassistant/components/device_tracker/__init__.py

@@ -426,12 +426,11 @@ class Device(Entity):
         if attributes:
             self._attributes.update(attributes)
 
-        self.gps = None
-
         if gps is not None:
             try:
                 self.gps = float(gps[0]), float(gps[1])
             except (ValueError, TypeError, IndexError):
+                self.gps = None
                 _LOGGER.warning('Could not parse gps value for %s: %s',
                                 self.dev_id, gps)