tests: payloading: push buffer before custom event

I'm improving appsrc to properly serialize custom events with the buffer flow, see gst-plugins-base#247 (closed)

This test was break as a side effect of this work as we were trying to push an event on a pad which did not receive a segment yet, raising: gstpad.c:4664:gst_pad_push_data:rtpgstpay0:src Got data flow before segment event

Fix it by pushing an empty buffer first so the segment is generated and propagated to the pad.