Fix join short circuit when probe is empty

[sopel39]

Join short circuit on empty probe does not work correctly. Previously (before join was ported to work processors) join operator was blocking on build side (lookupSourceProviderFuture) before getting first page.
Currently, it is still the case due to dynamic filter support. However, waiting for dynamic filters should be correctly solved by: #3414. When #3414 lands, it would be possible to eliminate lookupSourceProviderFuture.isDone() from:

    @Override
    public boolean needsInput()
    {
        return lookupSourceProviderFuture.isDone() && pageBuffer.isEmpty() && !pageBuffer.isFinished();
    }

This would allow to simplify join (make it plain WorkProcessorOperator) and to correctly support short circuiting on empty probe side.

[rohangarg]

@sopel39 : can this issue be fixed now as you have explained - since #3414 has been merged to master?

[sopel39]

I've a draft commits that simplify join. However, I'm not sure if short circuit can be implemented that easily. This is because build side might required to be closed gracefully because of spill support. This requires investigation