fix MyPy check

Signed-off-by: kenwoodjw <blackxin55+@gmail.com>

api/core/workflow/nodes/knowledge_retrieval/knowledge_retrieval_node.py

@@ -375,17 +375,17 @@ class KnowledgeRetrievalNode(LLMNode):
                                 expected_value = self.graph_runtime_state.variable_pool.convert_template(
                                     expected_value
                                 ).value[0]
-                                if hasattr(expected_value, "value_type"):
+                                if hasattr(expected_value, "value_type") and expected_value is not None:
                                     if expected_value.value_type == "number":  # type: ignore
                                         expected_value = expected_value.value  # type: ignore
                                     elif expected_value.value_type == "string":  # type: ignore
                                         expected_value = re.sub(r"[\r\n\t]+", " ", expected_value.text).strip()  # type: ignore
-                                    elif expected_value.value_type in (
+                                    elif expected_value.value_type in (  # type: ignore
                                         "array[number]",
                                         "array[string]",
                                         "array[object]",
                                         "array",
-                                    ):  # type: ignore
+                                    ):
                                         expected_value = expected_value.value  # type: ignore
                                     else:
                                         raise ValueError("Invalid expected metadata value type")