filter/Filter: Flush() returns empty span, not nullptr

src/filter/Filter.hxx

@@ -49,7 +49,7 @@ public:
 
 	/**
 	 * Flush pending data and return it.  This should be called
-	 * repeatedly until it returns nullptr.
+	 * repeatedly until it returns an empty span.
 	 *
 	 * After calling this method, this object cannot be used again
          * (not even Reset() is allowed).

src/output/Thread.cxx

@@ -389,7 +389,7 @@ AudioOutputControl::InternalDrain() noexcept
 
 		while (true) {
 			auto buffer = source.Flush();
-			if (buffer.data() == nullptr)
+			if (buffer.empty())
 				break;
 
 			PlayFull(*output, buffer);

test/run_filter.cxx

@@ -128,7 +128,7 @@ try {
 
 	while (true) {
 		auto dest = filter->Flush();
-		if (dest.data() == nullptr)
+		if (dest.empty())
 			break;
 		output_fd.FullWrite(dest);
 	}