`edgedb instance link` should not use ctrl-D as an "accept defaults" keybinding

- EdgeDB Version: n/a
- EdgeDB CLI Version: `EdgeDB CLI 5.2.0-dev` (`ae5516f1f40e87e64c8ec848c6267644563c5899` - `master` as of 2024-05-13)
- OS Version: `NixOS 23.11.6827.383ffe076d9b`

steps to reproduce:

1. start an instance that is eligible for remote linking - eg `docker run -d --name edgedb52 -e EDGEDB_SERVER_SECURITY=insecure_dev_mode -p 5656:5656 edgedb/edgedb:5.2`
2. link it using the CLI: `edgedb -P 5656 instance link --trust-tls-cert docker-52`

current behavior:

```
> edgedb -P 5656 instance link --trust-tls-cert docker-52
Specify server host [default: localhost]:
> localhost
Specify database user [default: edgedb]:
> edgedb
Specify database/branch (CTRL + D for default):
>
edgedb error: InvalidArgumentError: invalid database: empty string
```

for the first two questions, we use `` to accept the default option; for the third we expect ``, and error out if the user hits enter again. ctrl-D is in my muscle memory as "close this terminal" and it's weird to overload it.

desired behavior:

we should be consistent about expecting `` as the "accept the default" when prompting users for values