REST binary sensor value_template optional (#8596)

According to the documentation, the `value_template` for the REST
binary_sensor is not required. However, if you don't provide this when
setting up a binary sensor, the component fails. Looks like a variable
was not being set, which I've now included.

This should make the REST binary sensor act the same way as the REST
sensor now.

homeassistant/components/binary_sensor/rest.py

@@ -107,6 +107,8 @@ def is_on(self):
         if self.rest.data is None:
             return False
 
+        response = self.rest.data
+
         if self._value_template is not None:
             response = self._value_template.\
                 async_render_with_possible_json_value(self.rest.data, False)