Add NULL as a first class citizen of the Unity Container

src/Strategies/LifetimeStrategy.cs

@@ -65,7 +65,7 @@ namespace Unity.Strategies
                 context.RequiresRecovery = recoveryPolicy;
 
             var existing = policy.GetValue(context.Lifetime);
-            if (existing != null)
+            if (LifetimeManager.NoValue != existing)
             {
                 context.Existing = existing;
                 context.BuildComplete = true;

src/UnityContainer.IUnityContainer.cs

@@ -114,7 +114,7 @@ namespace Unity
             try
             {
                 // Validate input
-                if (null == instance) throw new ArgumentNullException(nameof(instance));
+                if (null == typeFrom) throw new InvalidOperationException($"At least one of Type arguments '{nameof(type)}' or '{nameof(instance)}' must be not 'null'");
 
                 if (null == lifetimeManager) lifetimeManager = new ContainerControlledLifetimeManager();
                 if (((LifetimeManager)lifetimeManager).InUse) throw new InvalidOperationException(LifetimeManagerInUse);

src/UnityContainer.Resolution.cs

@@ -440,7 +440,7 @@ namespace Unity
 
                     var lifetimeManager = (LifetimeManager)parentRef.Get(typeof(LifetimeManager));
                     var result = lifetimeManager?.GetValue();
-                    if (null != result) return result;
+                    if (LifetimeManager.NoValue != result) return result;
 
                     throw new InvalidOperationException($"Circular reference for Type: {parentRef.Type}, Name: {parentRef.Name}",
                             new CircularDependencyException());