refactor: add tip when model_dict is None

Closes #628

Author: SWHL

python/rapidocr/inference_engine/base.py

@@ -1,4 +1,4 @@
-    # -*- encoding: utf-8 -*-
+# -*- encoding: utf-8 -*-
 # @Author: SWHL
 # @Contact: liekkaskono@163.com
 import abc
@@ -113,6 +113,11 @@ def get_model_url(cls, file_info: FileInfo) -> Dict[str, str]:
             cls.model_info, f"{engine_type}.{ocr_version}.{task_type}"
         )
 
+        if not model_dict:
+            raise ValueError(
+                f"Unsupported configuration: {engine_type}.{ocr_version}.{task_type}.{model_type}"
+            )
+
         # 优先查找 server 模型
         if model_type == ModelType.SERVER.value:
             for k in model_dict: