fix: add missing return statement

Author: christolis

application/src/main/java/org/togetherjava/tjbot/features/chatgpt/ChatGptService.java

@@ -178,5 +178,7 @@ private Optional<String> getMessageResponse(String message) {
             logger.warn("There was an error using the OpenAI API: {}",
                     runtimeException.getMessage());
         }
+
+        return Optional.empty();
     }
 }