This was the default behavior, because all members with `user_id` set to
`null` were excluded.

For each member being rendered on
Projects::ProjectMembersController#index, there was a query to the users
table to retrieve the corresponding user:

```
SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1
```

Realized window functions are not available in older MySQL versions
either.

Falling back to DISTINCT ON for postgresql and a convoluted translation
for MySQL.

Closes #41461.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixed more tests