U4-7807 Domain, language, public access cache (GetAll) caches not working when there are no items

src/Umbraco.Core/Cache/FullDataSetRepositoryCachePolicy.cs

@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using Umbraco.Core.Collections;
 using Umbraco.Core.Models.EntityBase;
@@ -12,10 +14,18 @@ namespace Umbraco.Core.Cache
     internal class FullDataSetRepositoryCachePolicy<TEntity, TId> : DefaultRepositoryCachePolicy<TEntity, TId>
         where TEntity : class, IAggregateRoot
     {
-        public FullDataSetRepositoryCachePolicy(IRuntimeCacheProvider cache) : base(cache, new RepositoryCachePolicyOptions())
+        public FullDataSetRepositoryCachePolicy(IRuntimeCacheProvider cache) : base(cache,
+            new RepositoryCachePolicyOptions
+            {
+                //Definitely allow zero'd cache entires since this is a full set, in many cases there will be none,
+                // and we must cache this!
+                GetAllCacheAllowZeroCount = true
+            })
         {
         }
 
+        private bool? _hasZeroCountCache;
+
         /// <summary>
         /// For this type of caching policy, we don't cache individual items
         /// </summary>
@@ -44,6 +54,19 @@ namespace Umbraco.Core.Cache
             });
         }
 
+        /// <summary>
+        /// Looks up the zero count cache, must return null if it doesn't exist
+        /// </summary>
+        /// <returns></returns>
+        protected override bool HasZeroCountCache()
+        {
+            if (_hasZeroCountCache.HasValue)
+                return _hasZeroCountCache.Value;
+
+            _hasZeroCountCache = Cache.GetCacheItem<DeepCloneableList<TEntity>>(GetCacheTypeKey()) != null;
+            return _hasZeroCountCache.Value;
+        }
+
         /// <summary>
         /// This policy will cache the full data set as a single collection
         /// </summary>
@@ -51,6 +74,10 @@ namespace Umbraco.Core.Cache
         protected override TEntity[] GetAllFromCache()
         {
             var found = Cache.GetCacheItem<DeepCloneableList<TEntity>>(GetCacheTypeKey());
+            
+            //This method will get called before checking for zero count cache, so we'll just set the flag here
+            _hasZeroCountCache = found != null;
+
             return found == null ? new TEntity[] { } : found.WhereNotNull().ToArray();
         }
     }