fix: (external) avoid NullReferenceException on ImportReferences call (#1434)

* fix: NullReferenceException during ImportReferences in
NetworkBehaviourILPP (#1378)

In some cases, for example when assembly is System.Colletions.Immutable, null value is passed to .Add() method causing NullReferenceException
during the build. Simply guarding against it solves the issue.

Co-authored with @JadenH
Co-authored-by: rafek <rafal.legiedz@gmail.com>

Author: andrews-unity

com.unity.netcode.gameobjects/Editor/CodeGen/NetworkBehaviourILPP.cs

@@ -312,7 +312,11 @@ private bool ImportReferences(ModuleDefinition moduleDefinition)
             assemblies.Add(m_MainModule.Assembly);
             foreach (var reference in m_MainModule.AssemblyReferences)
             {
-                assemblies.Add(m_AssemblyResolver.Resolve(reference));
+                var assembly = m_AssemblyResolver.Resolve(reference);
+                if (assembly != null)
+                {
+                    assemblies.Add(assembly);
+                }
             }
 
             var extensionConstructor =