non-IMMUTABLE functions are not allowed in CASE or COALESCE statements

Executing this SQL gives an error on a sharded database (table is distributed on root_id):

```
UPDATE sample_table set amount = 50
WHERE id = '2a10f3d8-e492-4582-9d35-f731e91fc937'
and root_id = 1
AND now() between coalesce(start_date, '1-1-1900') and coalesce(end_date, now() + interval '1' day)
```

```
ERROR: non-IMMUTABLE functions are not allowed in CASE or COALESCE statements
```

If I run this on a plain old PSQL instance, it executes just fine.
There's probably better ways to determine if now is between start and end (or if they're null), of course. Though it does seem like a common pattern.