videorate: fix assertion when pushing last and only buffer without duration

videorate: fix assertion when pushing last and only buffer without duration

Fixing this pipeline:
  gst-launch-1.0 filesrc location=sample.png ! pngdec ! videorate ! fakesink

- videorate receives a single buffer with pts = 0, duration = invalid;
- then it receives eos triggering this buffer to be pushed downstream;
- the pushing code was assuming that a duration was set, which is
  impossible as we received a single buffer and no output framerate was
  set either. So the best we can do is to push the buffer without
  duration.

Fix #1177

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2296>