Make sure we always create a valid index if the database has old/invalid json data

src/GitHub.App/Caches/CacheIndex.cs

@@ -62,6 +62,7 @@ public static IObservable<CacheIndex> AddAndSaveToIndex(IBlobCache cache, string
             Guard.ArgumentNotNull(item, nameof(item));
 
             return cache.GetOrCreateObject(indexKey, () => Create(indexKey))
+                .Select(x => x.IndexKey == null ? Create(indexKey) : x)
                 .Do(index =>
                 {
                     var k = string.Format(CultureInfo.InvariantCulture, "{0}|{1}", index.IndexKey, item.Key);

src/GitHub.App/Extensions/AkavacheExtensions.cs

@@ -200,7 +200,11 @@ static IObservable<T> GetAndFetchLatestFromIndex<T>(this IBlobCache This,
             bool shouldInvalidateOnError = false)
                 where T : CacheItem
         {
-            var idx = Observable.Defer(() => This.GetOrCreateObject(key, () => CacheIndex.Create(key))).Replay().RefCount();
+            var idx = Observable.Defer(() => This
+                                            .GetOrCreateObject(key, () => CacheIndex.Create(key)))
+                                            .Select(x => x.IndexKey == null ? CacheIndex.Create(key) : x)
+                                            .Replay()
+                                            .RefCount();
 
 
             var fetch = idx
@@ -264,6 +268,7 @@ public static IObservable<T> PutAndUpdateIndex<T>(this IBlobCache blobCache,
             {
                 var absoluteExpiration = blobCache.Scheduler.Now + maxCacheDuration;
                 return blobCache.GetOrCreateObject(key, () => CacheIndex.Create(key))
+                    .Select(x => x.IndexKey == null ? CacheIndex.Create(key) : x)
                     .SelectMany(index => fetchFunc()
                             .Catch<T, Exception>(Observable.Throw<T>)
                             .SelectMany(x => x.Save<T>(blobCache, key, absoluteExpiration))