Streaming generate_series should work with unknown as start

[stdrc]

Seems we cannot correctly infer streaming generate_series with now() when the start argument is an unknown, as found by @kwannoel here: https://github.com/risingwavelabs/risingwave-docs/pull/2637#issuecomment-2373942740

dev=> create materialized view m1 as SELECT time::timestamptz FROM generate_series(
  '2020-01-01 00:00:00',
  now(),
  interval '1 hour'
) t(time);
ERROR:  Failed to run the query

Caused by:
  function generate_series(unknown, timestamp with time zone, interval) does not exist

Users have to do casting like '2020-01-01 00:00:00'::timestamptz.

This is not intended, should be fixed.