baseparse: only post 'no valid frames' error if buffers were received

baseparse: only post 'no valid frames' error if buffers were received

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