fix: Fixes an ILPP crash when a NetworkBehaviour had a field of a type with no base type (an interface or object) (#1914)

Author: ShadauxCat

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

@@ -114,6 +114,10 @@ public override ILPostProcessResult Process(ICompiledAssembly compiledAssembly)
                                     // Get the field type and its base type
                                     var fieldType = field.FieldType;
                                     var baseType = fieldType.Resolve().BaseType;
+                                    if (baseType == null)
+                                    {
+                                        continue;
+                                    }
                                     // This type stack is used for resolving NetworkVariableSerialization's T value
                                     // When looking at base types, we get the type definition rather than the
                                     // type reference... which means that we get the generic definition with an