fix(llm-node): handle NoneSegment variables properly (#9978)

Author: laipz8200

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

@@ -327,7 +327,7 @@ def _fetch_inputs(self, node_data: LLMNodeData) -> dict[str, Any]:
             if variable is None:
                 raise ValueError(f"Variable {variable_selector.variable} not found")
             if isinstance(variable, NoneSegment):
-                continue
+                inputs[variable_selector.variable] = ""
             inputs[variable_selector.variable] = variable.to_object()
 
         memory = node_data.memory