Fix settings of MQTT topic prepend (#274)

* Fix settings of MQTT topic prepend

* Update ingest/api/send_mqtt.py

Co-authored-by: Jukka Pelli <132542798+fjugipe@users.noreply.github.com>

* Update ingest/api/send_mqtt.py

Co-authored-by: Jukka Pelli <132542798+fjugipe@users.noreply.github.com>

* FIx missing bracket

---------

Co-authored-by: Jukka Pelli <132542798+fjugipe@users.noreply.github.com>

Author: Teddy-1000

ingest/api/send_mqtt.py

@@ -6,8 +6,11 @@
 
 logger = logging.getLogger(__name__)
 
-if "MQTT_TOPIC_PREPEND" in os.environ:
-    mqtt_topic_prepend = os.getenv("MQTT_TOPIC_PREPEND", "")
+mqtt_topic_prepend = os.getenv("MQTT_TOPIC_PREPEND", "")
+if not mqtt_topic_prepend or mqtt_topic_prepend == "/":
+    mqtt_topic_prepend = ""
+    logger.error("MQTT_TOPIC_PREPEND cannot be just a '/'. Setting topic prepend to empty string.")
+else:
     mqtt_topic_prepend = mqtt_topic_prepend if mqtt_topic_prepend.endswith("/") else mqtt_topic_prepend + "/"