Return graphql.Null in interface resolver when passed a typed nil

Go's dreaded _typed nil_ strikes again. Nil pointers of struct types
aren't equal to nil interface pointers.

See https://golang.org/doc/faq#nil_error

codegen/interface.gotpl

@@ -10,6 +10,9 @@ func (ec *executionContext) _{{$interface.Name}}(ctx context.Context, sel ast.Se
 				return ec._{{$implementor.Name}}(ctx, sel, &obj)
 		{{- end}}
 		case *{{$implementor.Type | ref}}:
+			if obj == nil {
+				return graphql.Null
+			}
 			return ec._{{$implementor.Name}}(ctx, sel, obj)
 	{{- end }}
 	default:

codegen/testserver/interfaces.graphql

@@ -1,6 +1,7 @@
 extend type Query {
     shapes: [Shape]
     noShape: Shape @makeNil
+    noShapeTypedNil: Shape @makeTypedNil
 }
 
 interface Shape {
@@ -18,3 +19,4 @@ type Rectangle implements Shape {
 union ShapeUnion @goModel(model:"testserver.ShapeUnion") = Circle | Rectangle
 
 directive @makeNil on FIELD_DEFINITION
+directive @makeTypedNil on FIELD_DEFINITION

codegen/testserver/interfaces_test.go

@@ -40,4 +40,28 @@ func TestInterfaces(t *testing.T) {
 		var resp interface{}
 		c.MustPost(`{ noShape { area } }`, &resp)
 	})
+
+	t.Run("interfaces can be typed nil", func(t *testing.T) {
+		resolvers := &Stub{}
+		resolvers.QueryResolver.NoShapeTypedNil = func(ctx context.Context) (shapes Shape, e error) {
+			panic("should not be called")
+		}
+
+		srv := handler.GraphQL(
+			NewExecutableSchema(Config{
+				Resolvers: resolvers,
+				Directives: DirectiveRoot{
+					MakeTypedNil: func(ctx context.Context, obj interface{}, next graphql.Resolver) (res interface{}, err error) {
+						var circle *Circle
+						return circle, nil
+					},
+				},
+			}),
+		)
+
+		c := client.New(srv)
+
+		var resp interface{}
+		c.MustPost(`{ noShapeTypedNil { area } }`, &resp)
+	})
 }

codegen/testserver/resolver.go

@@ -197,6 +197,9 @@ func (r *queryResolver) Shapes(ctx context.Context) ([]Shape, error) {
 func (r *queryResolver) NoShape(ctx context.Context) (Shape, error) {
 	panic("not implemented")
 }
+func (r *queryResolver) NoShapeTypedNil(ctx context.Context) (Shape, error) {
+	panic("not implemented")
+}
 func (r *queryResolver) Issue896a(ctx context.Context) ([]*CheckIssue896, error) {
 	panic("not implemented")
 }