flink won't return first_value() of NULL

[nparrish42]

checking this in as window_first_value.sql, but here's the simplest repro:

regression=# create table tiny (
regression(# id integer,
regression(#     a integer,
regression(#     b integer,
regression(#     c integer,
regression(#     PRIMARY KEY (id)
regression(# );
CREATE TABLE
regression=# insert into tiny values (1, 1, 1, NULL), (2, 1, 2, 66);
INSERT 0 2
regression=# select  a,
regression-#         b,
regression-#         c,
regression-#         first_value(c) over (partition by a order by b) as first_c
regression-# from tiny;
 a | b | c  | first_c
---+---+----+---------
 1 | 1 |    |
 1 | 2 | 66 |
(2 rows)

Flink SQL> select  a,
>         b,
>         c,
>         first_value(c) over (partition by a order by b) as first_c
> from tiny;
+----+-------------+-------------+-------------+-------------+
| op |           a |           b |           c |     first_c |
+----+-------------+-------------+-------------+-------------+
| +I |           1 |           2 |          66 |          66 |
| -D |           1 |           2 |          66 |          66 |
| +I |           1 |           1 |      <NULL> |      <NULL> |
| +I |           1 |           2 |          66 |          66 |
+----+-------------+-------------+-------------+-------------+
Received a total of 4 rows

first_c should always be NULL.