sctpenc: check if dataqueue is empty
sctpenc: check if dataqueue is empty

When transitioning from PLAYING to READY, the queue can be empty.
If the queue is empty, 'gst_data_queue_pop' will be blocked.

Closes #62

sancane/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins