fix(api): fix single stepping variable loading (#25908)

api/core/workflow/nodes/llm/node.py

                 model_instance=model_instance,
             )
 
-            query = None
+            query: str | None = None
             if self._node_data.memory:
                 query = self._node_data.memory.query_prompt_template
                 if not query and (

api/core/workflow/workflow_entry.py

             input_value = user_inputs.get(node_variable)
             if not input_value:
                 input_value = user_inputs.get(node_variable_key)
+            if input_value is None:
+                continue
 
             if isinstance(input_value, dict) and "type" in input_value and "transfer_method" in input_value:
                 input_value = file_factory.build_from_mapping(mapping=input_value, tenant_id=tenant_id)