In #41549 a user was getting an error that the `pool_config` was `nil`
so the `all_connection_pools` method would raise an error. After getting
a reproduction application I saw that if the application is
misconfigured this can happen. For example, if an application uses
`:all` for the writing role but does not set
`config.active_record.writing_role = :all` then the
`setup_shared_connection_pool` pool method will get a `nil` value for
the `writing_pool_config` and set that in `set_pool_config`. I
considered fixing this in `setup_shared_connection_pool` directly and
raising an error there, but there's a possibility this _can_ happen in
an external gem or application code if they're using these private APIs
and realistically we never want any code, Rails or otherwise, to be able
to set a `nil` pool config in the pools.

Note: In the test I made a new connection handler so that we have
isolated pools to test against. Otherwise we'll be testing against the
existing pools but we don't want to mess those up.

Closes #41549