dvrReadBufferQueueDequeue: allow empty metadata

Currently, when there is no metadata on the read buffer queue, we have
to pass in dummy out_metadata; otherwise the API always return
-EINVAL. This CL allows (nullptr, 0) as legit input and add test to
cover that.

Bug: 62958474
Test: Ran the updated DvrBufferQueueTest
Change-Id: I0ad666006a5d93b67a48b9c34f27e325dfa9a419