fix: credential *** should be string (#8785)

api/core/tools/provider/tool_provider.py

@@ -153,6 +153,9 @@ class ToolProviderController(BaseModel, ABC):
 
             # check type
             credential_schema = credentials_need_to_validate[credential_name]
+            if not credential_schema.required and credentials[credential_name] is None:
+                continue
+
             if credential_schema.type in {
                 ToolProviderCredentials.CredentialsType.SECRET_INPUT,
                 ToolProviderCredentials.CredentialsType.TEXT_INPUT,