Ignore nextVisit messages with no instrument.

Such messages are created by summit scripts that don't take
observations (e.g., a standalone telescope slew command), and they
confuse Prompt Processing (which assumes that having the wrong
instrument is a serious error).

src/main.py

@@ -254,6 +254,11 @@ async def main() -> None:
 
                     logging.info(f"message deserialized {next_visit_message_initial}")
 
+                    if not next_visit_message_initial["message"]["instrument"]:
+                        logging.info("Message does not have an instrument. Assuming "
+                                     "it's not an observation.")
+                        continue
+
                     next_visit_message_updated = NextVisitModel(
                         salIndex=next_visit_message_initial["message"]["salIndex"],
                         scriptSalIndex=next_visit_message_initial["message"][