code: def is_missing_variable(
      value_node: ValueNode, variables: Dict[str, Any] = None
  ) -> bool:
      
      return isinstance(value_node, VariableNode) and (
          not variables or is_invalid(variables.get(value_node.name.value, INVALID))
      )
docs: Check if `value_node` is a variable not defined in the `variables` dict.