Check if awaiting collection is empty before waiting on condition in conf height processor

[wezrule]

@guilhermelawless found that the confirmation_height.unbounded_block_cache_iteration test failed intermittently.

It was because a check is made to see if the awaiting_processing collection is empty before waiting on the condition variable. However the lock is unlocked before this action is made (while other actions are performed) and it's possible confirmation_height_processor::add gets called (which notifies the condition), however we condition_variable::wait after this, so it ends up not being processed until another block is confirmed.