fix test_timedelta_cumsum

Author: TrevorBergeron

bigframes/core/rewrite/timedeltas.py

@@ -111,6 +111,8 @@ def _rewrite_expressions(expr: ex.Expression, schema: schema.ArraySchema) -> _Ty
 
 
 def _rewrite_scalar_constant_expr(expr: ex.ScalarConstantExpression) -> _TypedExpr:
+    if expr.value is None:
+        return _TypedExpr(ex.const(None, expr.dtype), expr.dtype)
     if expr.dtype == dtypes.TIMEDELTA_DTYPE:
         int_repr = utils.timedelta_to_micros(expr.value)  # type: ignore
         return _TypedExpr(ex.const(int_repr, expr.dtype), expr.dtype)