src/stats: support checking if the test_stats_queue is empty
src/stats: support checking if the test_stats_queue is empty

We need to know if we have processed all collected stats from the queue, so
return NULL in that case.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

ejoerns/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc