chore: avoid implicit optional in type annotations of method (#8727)

api/core/tools/tool/builtin_tool.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 from core.model_runtime.entities.llm_entities import LLMResult
 from core.model_runtime.entities.message_entities import PromptMessage, SystemPromptMessage, UserPromptMessage
 from core.tools.entities.tool_entities import ToolProviderType
@@ -124,7 +126,7 @@ class BuiltinTool(Tool):
 
         return result
 
-    def get_url(self, url: str, user_agent: str = None) -> str:
+    def get_url(self, url: str, user_agent: Optional[str] = None) -> str:
         """
         get url
         """