fix broken migration abstraction, fixes restart after upgrade so the runtimestate is correct, fixes sql alter column migration to actually run if its nullable.

src/Umbraco.Core/Configuration/Models/NuCacheSerializerType.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Umbraco.
+// Copyright (c) Umbraco.
 // See LICENSE for more details.
 
 namespace Umbraco.Cms.Core.Configuration.Models
@@ -8,8 +8,7 @@ namespace Umbraco.Cms.Core.Configuration.Models
     /// </summary>
     public enum NuCacheSerializerType
     {
-        JSON,
-
-        MessagePack
+        MessagePack = 1, // Default
+        JSON = 2
     }
 }

src/Umbraco.Core/Configuration/Models/NuCacheSettings.cs

@@ -13,12 +13,10 @@ namespace Umbraco.Cms.Core.Configuration.Models
         /// </summary>
         public int? BTreeBlockSize { get; set; }
 
-        // TODO: Set default to MessagePack
-
         /// <summary>
         /// The serializer type that nucache uses to persist documents in the database.
         /// </summary>
-        public NuCacheSerializerType NuCacheSerializerType { get; set; }
+        public NuCacheSerializerType NuCacheSerializerType { get; set; } = NuCacheSerializerType.MessagePack;
 
         /// <summary>
         /// The paging size to use for nucache SQL queries.