FileCache throws NullReferenceException

[SoundersFan]

FileCache.InitializeTypeAsync(Stream...) should not return null. It should return Task.FromResult<StorageFile>(null). Otherwise, CacheBase attempts to dereference the Task to call ConfigureAwait(false) which results in a NullReferenceException.

Fixes #3104

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

If the Uri has not already been cached, calling the following throws a NullReferenceException

StorageFile file = await FileCache.Instance.GetFromCacheAsync(new Uri("https://www.bing.com"));

What is the new behavior?

No NullReferenceException is thrown.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

I also updated the what the InitializeTypeAsync(StorageFile...) return to be consistent with the other InitializeTypeAsync(Stream...) return. In the end, Task.FromResult is the more appropriate thing to return when your asynchronous method does not await.

[ghost]

Thanks for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[hermitdave]

Please rebase so the branch is up to date. Looks good otherwise

[michael-hawker]

Thanks @hermitdave for taking a look and @SoundersFan for the contribution!