pipewire: don't drain empty streams

Fixes the following assert when seeking more than one time past the end of
the file.

src/audio_output/dec.c:1107: vlc_aout_stream_Drain: Assertion `!atomic_load_explicit(&stream->drained, memory_order_relaxed)' failed.

Calling `pw_stream_flush(s->stream, true)`, when empty, leads to
undefined behavior and might cause the `stream_drained()` callback to be
called in loop (leading to the mentioned assert after a flush).