Ignore empty cache and don't cache invalid stream

[PureWeen]

Description of Change

• If the stream returned from the HttpClient is invalid the code was still creating an empty cache file which would cause subsequent calls to just use an empty file for the image instead of retrying to stream
• If the cached file is empty then don't use it and just retry the remote stream.

Issues Resolved

• fixes ImageLoaderSourceHandler: Could not retrieve image or image data was invalid: Uri #7751

Platforms Affected

• Core/XAML (all platforms)
• iOS
• Android
• UWP

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

• Download the nuget
• Run this sample https://docs.microsoft.com/en-us/samples/xamarin/xamarin-forms-samples/workingwithimages/ against 4.1.0.709244 which will load an invalid image into cache
• now rerun the sample with the nuget that's part of this sample
• the image should now load

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[serbesthamit]

Goto Android Project, then click on properties,
click on assemblyinfo.cs
[assembly: Application(UsesCleartextTraffic = true)]

https://forums.xamarin.com/discussion/164771/cleartext-http-traffic-not-permitted