Avoid adding empty providers

[Richasy]

Fixes #190

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

When adding a WebAccountProvider, there is no null judgment on the acquired provider, resulting in a null reference in the provider added to the list, which directly causes an unexpected provider (null) to be passed in when the WebAccountProviderCommand is created, causing the process to be interrupted and unable to log in.

What is the new behavior?

Create a new method and add it to the list only when the obtained WebAccountProvider is not null.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• 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

[shweaver-MSFT]

Need to clean up the build error, but otherwise looks good to me :)

[michael-hawker]

Looks like it may have been a NuGet timeout trying to restore packages, reran the job, so we'll see what happens.