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

src/output/plugins/AlsaOutputPlugin.cxx

@@ -872,7 +872,7 @@ 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())
 		return size;
 
 	std::unique_lock<Mutex> lock(mutex);

src/output/plugins/OSXOutputPlugin.cxx

@@ -887,7 +887,7 @@ OSXOutput::Play(const void *chunk, size_t size)
 #ifdef ENABLE_DSD
         if (dop_enabled) {
 		const auto e = pcm_export->Export({chunk, size});
-		if (e.size == 0)
+		if (e.empty())
 			return size;
 
 		size_t bytes_written = ring_buffer->push((const uint8_t *)e.data, e.size);