fix: Fix error with nil value on structure types (#1380)

* Fix no cond clause evaluated to a truthy value error

* Fix error with nil value

* Add tests

lib/ash/type/struct.ex

@@ -246,6 +246,8 @@ defmodule Ash.Type.Struct do
     end
   end
 
+  defp handle_instance_of(nil, _), do: {:ok, nil}
+
   defp handle_instance_of(value, constraints) do
     case Keyword.fetch(constraints, :instance_of) do
       {:ok, struct} ->
@@ -256,7 +258,7 @@ defmodule Ash.Type.Struct do
           is_struct(value) ->
             :error
 
-          value ->
+          true ->
             if constraints[:fields] do
               {:ok, struct(struct, value)}
             else

test/type/struct_test.exs

@@ -17,7 +17,14 @@ defmodule Type.StructTest do
 
     actions do
       default_accept :*
-      defaults [:read, :destroy, create: :*, update: :*]
+      defaults [:read, :destroy, update: :*]
+
+      create :create do
+        primary? true
+        accept [:*]
+
+        argument :dummy_metadata, :struct, constraints: [instance_of: Metadata], allow_nil?: true
+      end
     end
 
     attributes do
@@ -185,4 +192,12 @@ defmodule Type.StructTest do
              }
            ] = changeset.errors
   end
+
+  test "it handles nil argument" do
+    changeset =
+      Post
+      |> Ash.Changeset.for_create(:create, %{dummy_metadata: nil})
+
+    assert changeset.valid?
+  end
 end