docs: remove @empty <li> from the a11y tree

[alejandrocuba]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The existing examples of the @empty block renders an empty unordered list with a list item containing the text “The list is empty”. This is not a good experience for users with accessibility needs, as pointed out by @ahasall.

Issue Number: N/A

What is the new behavior?

A <li [attr.aria-hidden]="true"> element is now rendered within the @empty block, as the resulting node will be removed from the accessibility tree in the browser.

Does this PR introduce a breaking change?

• Yes
• No

Other information