Added an empty list example to the ListView docs

[darrenaustin]

Description

Added an example to the docs for ListView that illustrate how to handle presenting a custom UI when the list is empty.

Related Issues

#41191

Tests

Just a doc change, so no tests were added or updated.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them?

• No, no existing tests failed, so this is not a breaking change.

[HansMuller]

LGTM