Support returning nulls from slices

codegen/testserver/resolver.go

@@ -125,6 +125,9 @@ func (r *queryResolver) Panics(ctx context.Context) (*Panics, error) {
 func (r *queryResolver) DefaultScalar(ctx context.Context, arg string) (string, error) {
 	panic("not implemented")
 }
+func (r *queryResolver) Slices(ctx context.Context) (*Slices, error) {
+	panic("not implemented")
+}
 func (r *queryResolver) ValidType(ctx context.Context) (*ValidType, error) {
 	panic("not implemented")
 }

codegen/testserver/slices.graphql

@@ -0,0 +1,10 @@
+extend type Query {
+    slices: Slices
+}
+
+type Slices {
+  test1: [String]
+  test2: [String!]
+  test3: [String]!
+  test4: [String!]!
+}

codegen/testserver/slices_test.go

@@ -0,0 +1,33 @@
+package testserver
+
+import (
+	"context"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/99designs/gqlgen/client"
+	"github.com/99designs/gqlgen/handler"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSlices(t *testing.T) {
+	resolvers := &Stub{}
+
+	srv := httptest.NewServer(handler.GraphQL(NewExecutableSchema(Config{Resolvers: resolvers})))
+	c := client.New(srv.URL)
+
+	t.Run("nulls vs empty slices", func(t *testing.T) {
+		resolvers.QueryResolver.Slices = func(ctx context.Context) (slices *Slices, e error) {
+			return &Slices{}, nil
+		}
+
+		var resp struct {
+			Slices Slices
+		}
+		c.MustPost(`query { slices { test1, test2, test3, test4 }}`, &resp)
+		require.Nil(t, resp.Slices.Test1)
+		require.Nil(t, resp.Slices.Test2)
+		require.NotNil(t, resp.Slices.Test3)
+		require.NotNil(t, resp.Slices.Test4)
+	})
+}

codegen/type.gotpl

@@ -65,6 +65,11 @@
 			{{- if $type.SelfMarshalling }}
 				return v
 			{{- else if $type.IsSlice }}
+				{{- if not $type.GQL.NonNull }}
+					if v == nil {
+						return graphql.Null
+					}
+				{{- end }}
 				ret := make(graphql.Array, len(v))
 				{{- if not $type.IsScalar }}
 					var wg sync.WaitGroup