Protect OverlapWindowPlugin against empty chunks

[JelleAalbers]

As discovered by @jorana, strax's OverlapWindowPlugin crashes with an IndexError if an empty chunk is found in the data stream. In general strax is not very robust against this case (see #162), but we are improving.

You can read how OverlapWindowPlugin works here. For an empty chunk, this PR changes strax to simply not send or discard any data; the input/output caches remain the same. The case of the last chunk (after which you must send everything even it if is empty) is already handled by the iter method.

This escaped the property-based testing due to a spurious condition in the test (I guess I just didn't consider empty chunks would be possible) that is now removed.

(This also removes some old but totally unrelated commented code)

[JoranAngevaare]

Thanks Jelle! It does solve the problem but causes problems elsewhere as it will keep waiting after which it crashes due to an strax.plugin.InputTimeoutExceeded error.

Could this be that the problem lies in that from line 52335ec#diff-db9a6da7a746e72e441939269037c9e6R126 there will never be an ‘end’ so it will not know that that chuck is finished (but just empty)?

[JelleAalbers]

InputTimeoutExceeded should come from the DAQReader, this is an exception raised as part of the online input support. I would guess this is an unrelated problem, e.g. maybe something is wrong with how that particular run ended?