Issue preloading soft deleted records

Nobody has claimed this yet.

I'll need to investigate this more, but we just ran in to an issue with some preloads.

Take this model setup

```crystal
class User < BaseModel
table do
# ...
column soft_deleted_at : Time?
has_many transactions : Transaction
end
end

class Transaction < BaseModel
table do
belongs_to user : User
end
end

class User::BaseQuery
include Avram::SoftDelete::Query

def initialize
defaults &.only_kept
end
end

class UserQuery < User::BaseQuery
end

class TransactionQuery < Transaction::BaseQuery
end
```

Then we had some code like this:

```crystal
# in the action
transactions = TransactionQuery.new.preload_user

# in the page

transactions.each do |transaction|
td do
link(transaction.user.username, to: Users::Show.with(transaction.user_id))
end
end
```

We kept getting a 500 error saying `NilAssertionError` which generally relates to calling `not_nil!` somewhere. In this case, our `user` should never be nil, but calling `transaction.user` says otherwise. We printed out the record, and saw this:

```
#
```

Looking up user 4, we saw the record existed, but it was soft deleted.

Avram should probably log some sort of warning that a required record could not be preloaded, or maybe when you preload an association that's been soft deleted, that record isn't returned at all?

Start with TransactionQuery.new.preload_user, transaction.user, and the Avram::SoftDelete::Query behavior shown in the report. Reproduce the preload of a soft-deleted User and trace how @_user_preloaded=true with @_preloaded_user=nil is handled. Done means the project has a decided and tested behavior for required associations whose records are soft deleted.

Written by the indexing model from the issue text.