Listener: Simplify assignment within accept()

We can just assign to the ret variable directly.

lib/Listener.cpp

@@ -91,14 +91,14 @@ void Listener::stop() {
 
 std::unique_ptr<Endpoint> Listener::accept() {
   std::unique_lock lk{m_queueLock};
-  if (m_endpointQueue.size()) {
-    std::unique_ptr<Endpoint> ret;
-    ret = std::move(m_endpointQueue.front());
+  if (m_endpointQueue.empty()) {
+    return nullptr;
+  }
+
+  auto ret = std::move(m_endpointQueue.front());
   m_endpointQueue.pop();
   return ret;
 }
-  return {};
-}
 
 Listener::Listener() = default;