fix: Don't throw when shutting down a config-less manager (#1479)

If shutting down a NetworkManager that doesn't have a NetworkConfig, it
would throw when trying to remove the OnTransportEvent delegate from the
transport.

Co-authored-by: Andrew Spiering <andrews@unity3d.com>

Author: simon-lemay-unity

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -1129,7 +1129,10 @@ internal void ShutdownInternal()
                 MessagingSystem = null;
             }
 
-            NetworkConfig.NetworkTransport.OnTransportEvent -= HandleRawTransportPoll;
+            if (NetworkConfig?.NetworkTransport != null)
+            {
+                NetworkConfig.NetworkTransport.OnTransportEvent -= HandleRawTransportPoll;
+            }
 
             if (SpawnManager != null)
             {

com.unity.netcode.gameobjects/Tests/Editor/StartStopTests.cs

@@ -52,6 +52,19 @@ public void TestFlagShutdown()
             Assert.False(m_NetworkManager.IsHost);
         }
 
+        [Test]
+        public void TestShutdownWithoutStartForExceptions()
+        {
+            m_NetworkManager.ShutdownInternal();
+        }
+
+        [Test]
+        public void TestShutdownWithoutConfigForExceptions()
+        {
+            m_NetworkManager.NetworkConfig = null;
+            m_NetworkManager.ShutdownInternal();
+        }
+
         [TearDown]
         public void Teardown()
         {