It is not used in any filters currently and is inherently evil. If
passing binary data to filters is required in the future, it should be
done with some AVOptions-based system.
It currently does the following:
1) get a zeroed audio buffer
2) copy some properties (but not the data) of the input buffer to it
3) pass this buffer to the output filter
This looks useless and is indeed not used by any filters, therefore
delete it.
Make ff_null_filter_samples() (just pass the buffer to the next filter)
the new default.
