Fix NullReferenceException in internal Proxify extension.

When a resource is requested for an unknonw id the Facade should return null. the extension method to create the proxy can not handle null values, however. The NullReferenceexception only occurs two layers deeper, where we correctly assume not to create a proxy for null.

Author: 1nf0rmagician

src/Moryx.Resources.Management/Facades/ResourceManagementFacade.cs

@@ -59,35 +59,35 @@ private void OnResourceRemoved(object sender, IPublicResource publicResource)
         public TResource GetResource<TResource>() where TResource : class, IPublicResource
         {
             ValidateHealthState();
-            return ResourceGraph.GetResource<TResource>().Proxify(TypeController);
+            return ResourceGraph.GetResource<TResource>()?.Proxify(TypeController);
         }
 
         public TResource GetResource<TResource>(long id)
             where TResource : class, IPublicResource
         {
             ValidateHealthState();
-            return ResourceGraph.GetResource<TResource>(id).Proxify(TypeController);
+            return ResourceGraph.GetResource<TResource>(id)?.Proxify(TypeController);
         }
 
         public TResource GetResource<TResource>(string name)
             where TResource : class, IPublicResource
         {
             ValidateHealthState();
-            return ResourceGraph.GetResource<TResource>(name).Proxify(TypeController);
+            return ResourceGraph.GetResource<TResource>(name)?.Proxify(TypeController);
         }
 
         public TResource GetResource<TResource>(ICapabilities requiredCapabilities)
             where TResource : class, IPublicResource
         {
             ValidateHealthState();
-            return ResourceGraph.GetResource<TResource>(r => requiredCapabilities.ProvidedBy(r.Capabilities)).Proxify(TypeController);
+            return ResourceGraph.GetResource<TResource>(r => requiredCapabilities.ProvidedBy(r.Capabilities))?.Proxify(TypeController);
         }
 
         public TResource GetResource<TResource>(Func<TResource, bool> predicate)
             where TResource : class, IPublicResource
         {
             ValidateHealthState();
-            return ResourceGraph.GetResource(predicate).Proxify(TypeController);
+            return ResourceGraph.GetResource(predicate)?.Proxify(TypeController);
         }
 
         public IEnumerable<TResource> GetResources<TResource>() where TResource : class, IPublicResource