[#18373] refactor the _normalize_python_tuple

Author: cchung100m

python/tvm/relax/block_builder.py

@@ -299,6 +299,9 @@ def _normalize_python_tuple(self, expr: Union[Expr, Sequence[Expr]]):
         """
         if isinstance(expr, (list, tuple)):
             return Tuple([self._normalize_python_tuple(element) for element in expr])
+        elif expr is None:
+            from . import op
+            return op.null_value()
         else:
             return expr