util/PeakBuffer: use IsEmpty() instead of IsNull()

The DynamicFifoBuffer methods never return nullptr when the buffer is
empty or full; instead, they return an empty buffer.  This bug caused
an endless loop.

src/event/FullyBufferedSocket.cxx

@@ -60,7 +60,7 @@ FullyBufferedSocket::Flush()
 	assert(IsDefined());
 
 	const auto data = output.Read();
-	if (data.IsNull()) {
+	if (data.IsEmpty()) {
 		IdleMonitor::Cancel();
 		CancelWrite();
 		return true;