Porting contribution from Kevin Labranche. Fix for 17111

--HG--
branch : 1.x

src/Orchard.Web/Core/Routable/ViewModels/RoutableEditorViewModel.cs

@@ -10,6 +10,7 @@ namespace Orchard.Core.Routable.ViewModels {
         [StringLength(1024)]
         public string Title { get; set; }
         [StringLength(1024)]
+        [Required(AllowEmptyStrings = false)]
         public string Slug { get; set; }
         public int? ContainerId { get; set; }
         public bool PromoteToHomePage { get; set; }