add support allowEmptySelection in select-popover

[osben]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this introduce a breaking change?

• Yes
• No

Other information

[averyjohnston]

Thanks for the PR! We appreciate the work you've done. However, we require all PRs to have an associated Github issue. For feature requests like this one, this ensures we have a use case to guide the design process.

If this is still something you're interested in seeing in the latest version of Ionic, I would encourage you to open an issue outlining your use case and any other details you can give. In the meantime, I'm going to close this out. Thank you!