Otherwise baseparse will consider empty streams to be an error while
an empty stream is a valid scenario. With this patch, errors would
only be emitted if the parser received data but wasn't able to
produce any output from it.
This change is only for push-mode operation as in pull mode an
empty file can be considered an error for the one driving the
pipeline
Includes a unit test for it
https://bugzilla.gnome.org/show_bug.cgi?id=733171
Baseparse stores buffers for reverse playback to push on the next
DISCONT, the issue was that it wouldn't ever check for a discont
on passthrough mode as it skips all real parsing. This test
was create to verify this issue and prevent it from happening again
https://bugzilla.gnome.org/show_bug.cgi?id=721941
