fix:优化代码逻辑

src/plugins/nonebot_bison/post/custom_post.py

@@ -31,13 +31,11 @@ class _CustomPost(BasePost):
             if message_segment.type == "text":
                 md += "{}<br>".format(message_segment.data.get("text", ""))
             elif message_segment.type == "image":
-                try:
-                    # 先尝试获取file的值，没有再尝试获取url的值，都没有则为空
-                    pic_res = message_segment.data.get(
-                        "file", message_segment.data.get("url", "")
-                    )
-                    assert pic_res
-                except AssertionError:
+                # 先尝试获取file的值，没有再尝试获取url的值，都没有则为空
+                pic_res = message_segment.data.get("file") or message_segment.data.get(
+                    "url", ""
+                )
+                if not pic_res:
                     logger.warning("无法获取到图片资源:MessageSegment.image中file/url字段均为空")
                 else:
                     md += "![Image]({})\n".format(pic_res)