Conditionally fires onDismiss when receiving ContentDialogResult::None

[rozele]

When the ContentDialog returns with a result of None, and there is no close button / neutral button option presented to the user, this change fires an onDismiss instead of attempting to fire the buttonNeutral.onPress callback, which is very unlikely to be registered.

It is very unlikely that RNW apps have leaned on this, but if someone is depending on the implementation detail of the "neutral" callback being called without presenting the button (by leaving the text for the neutral button option null), then this is a minor breaking change.

Fixes #8865

Microsoft Reviewers: Open in CodeFlow

[rozele]

As I said, here is an example where this is technically a breaking change:

Alert.alert('Alert', 'Warning, an alert is being shown!', [
  {onPress: () => setMessage('Neutral Pressed!')},
  {text: 'No'},
  {text: 'Yes'},
])

See before that the "Neutral Pressed!" log is displayed:

playground.2021-10-19.23-51-37.mp4

After, the callback is no longer invoked.

playground.2021-10-19.23-49-50.mp4

However, we now have onDismiss functionality, so the following code produces a "Dismissed!" log:

Alert.alert('Alert', 'Warning, an alert is being shown!', [
    {text: 'No'},
    {text: 'Yes'},
  ], {
    onDismiss: () => setMessage('Dismissed!'),
  })

playground.2021-10-19.23-48-28.mp4