get_shard_id_for_distribution_column returns 0 when it can't find a shard

- `NULL` would be the better thing to return.
- `get_shard_id_for_distribution_column` isn't called called by anything other than our tests (it's not called by enterprise or shard_rebalancer)
- `get_shard_id_for_distribution_column` is [publicly documented](https://docs.citusdata.com/en/v7.2/reference/user_defined_functions.html#get-shard-id-for-distribution-column)
- This is an interface change so we shouldn't do it for 7.3 but probably should do it for 8.0

Start with the tests that call get_shard_id_for_distribution_column and review the publicly documented user-defined function behavior. Check how the function handles a missing shard and confirm that enterprise and shard_rebalancer do not call it; done means the return behavior and corresponding tests and documentation reflect the agreed interface change.

Written by the indexing model from the issue text.