fix: default to ipv4address if listen address is null

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -24,6 +24,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed issue where server listen address was being assigned 127.0.0.1 when it was null.
 - Fixed issue where changes to a layer's weight would not synchronize unless a state transition was occurring.(#2399)
 - Fixed issue where `NetworkManager.LocalClientId` was returning the `NetworkTransport.ServerClientId` as opposed to the `NetworkManager.m_LocalClientId`. (#2398)
 - Fixed issue where a dynamically spawned `NetworkObject` parented under an in-scene placed `NetworkObject` would have its `InScenePlaced` value changed to `true`. This would result in a soft synchronization error for late joining clients. (#2396)

com.unity.netcode.gameobjects/Runtime/Transports/UTP/UnityTransport.cs

@@ -637,7 +637,7 @@ public void SetConnectionData(string ipv4Address, ushort port, string listenAddr
             {
                 Address = ipv4Address,
                 Port = port,
-                ServerListenAddress = listenAddress ?? string.Empty
+                ServerListenAddress = listenAddress ?? ipv4Address
             };
 
             SetProtocol(ProtocolType.UnityTransport);