Preview2 to Preview1 adapter hide repeated blocking-read calls

#### Feature

Change the adapter implementation to always ensure that `blocking_read` returning a `done` value of `true` will not call `blocking_read` again requiring an unnecessary empty vec, as this function call is effectively redundant.

#### Benefit

It removes redundancy in the read calls.

#### Implementation

Not sure, would be good to weigh up that it doesn't complicate the implementation.

#### Alternatives

It's fine to leave as-is for now, so this is certainly non-urgent, but it could also be nice to implement non-redundancy.