combine: Keep the timer active in the null mode only when running.
Previously thread_func() used PA_SINK_IS_OPENED() to check whether
some data should be rendered. process_render_null() used a different
check: it would return immediately if the sink was not in the RUNNING
state. This caused a busy loop when the sink was in the IDLE state,
because process_render_null() didn't update the timestamp, and
thread_func() still kept the timer active using the old timestamp.
pa_rtpoll_run() would return immediately because of the old timestamp.
This is fixed by using the same check in both thread_func() and
process_render_null(). Since the checks are the same, it's actually
redundant to have the check in process_render_null(), so it is now an
assertion.
BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=54779