FileCacheProvider - exception about invalid Manifest

[BAKAOLC]

What does the bug do?

I know there was a time when we had the same problem (https://github.com/TurnerSoftware/CacheTower/issues/264), but I still have this problem.

If there is no cache at the time of saving the manifest, attempting to add and access the cache will result in an exception:

System.NullReferenceException: Object reference not set to an instance of an object.
    at CacheTower.Providers.FileSystem.FileCacheLayer.GetAsync[T](String cacheKey) in /home/runner/work/CacheTower/CacheTower/src/CacheTower/Providers/FileSystem/FileCacheLayer.cs:line 211
    at CacheTower.CacheStack.GetAsync[T](String cacheKey) in /home/runner/work/CacheTower/CacheTower/src/CacheTower/CacheStack.cs:line 215

How can it be reproduced?

builder.AddFileCacheLayer(new("cache", new NewtonsoftJsonCacheSerializer(new()),
    TimeSpan.FromMilliseconds(300000)));

This doesn't even need to go through a reboot for this to occur after a certain amount of time.

Environment:

• Operating System: All (Windows, Mac and Linux)
• .NET Version: 9.0.101
• Nuget Package:
  • CacheTower 0.14.0
  • CacheTower.Providers.FileSystem.Json 0.14.0
  • CacheTower.Serializers.NewtonsoftJson 0.14.0