[Generator] BlankVideoGenerator causes pipeline to stuck on duration == 0

[DominikWolek]

Creating pipeline with

%BlankVideoGenerator{
  caps: %Membrane.Caps.Video.Raw{
      aligned: true,
      format: :I420,
      framerate: {4, 1},
      height: 360,
      width: 480
  },
  duration: 0
}

and CutAndMerge bin causes pipeline to stuck

[varsill]

I suspect that it might have been caused by the fact, that end_of_stream was not sent when start_of_stream hadn't been sent.
I couldn't reproduce this one on the newest membrane_core, where we have changed the end_of_stream behaviour in #577