output/{alsa,osx}: use ConstBuffer::empty()

src/output/plugins/AlsaOutputPlugin.cxx

@@ -939,7 +939,7 @@ AlsaOutput::Play(const void *chunk, size_t size)
 	assert(size % in_frame_size == 0);
 
 	const auto e = pcm_export->Export({chunk, size});
-	if (e.size == 0)
+	if (e.empty())
 		/* the DoP (DSD over PCM) filter converts two frames
 		   at a time and ignores the last odd frame; if there
 		   was only one frame (e.g. the last frame in the

src/output/plugins/OSXOutputPlugin.cxx

@@ -921,7 +921,7 @@ OSXOutput::Play(const void *chunk, size_t size)
 		   file), the result is empty; to avoid an endless
 		   loop, bail out here, and pretend the one frame has
 		   been played */
-		if (e.size == 0)
+		if (e.empty())
 			return size;
 
 		size_t bytes_written = ring_buffer->push((const uint8_t *)e.data, e.size);