add allowsEmptyCollection property to ComboBoxProps interface

[wdsjohn]

Workaround for #5690

It is already possible to provide allowsEmptyCollection as a prop to Combobox, but TypeScript will not be happy since its untyped.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Start storybook
Add allowsEmptyCollection={true} to e.g. ComboBoxAsyncLoadingExample story
Type in foo
Add 'f'
Remove 'f' and see that the menu remains open

If you set allowsEmptyCollection={false} or simply remove the prop you can see that when you remove 'f' that the menu will remain closed.

[LFDanLu]

Thanks for the PR! I will be hopefully revisiting ComboBox soon and will let you know if we decide to expose this prop or not, but in the meantime would you mind signing the CLA: https://react-spectrum.adobe.com/contribute.html#contributor-license-agreement? That will get the build step passing

[reidbarber]

GET_BUILD

[LFDanLu]

I think this should be fine as a stopgap solution until we can spend some time revisiting the internal logic for combobox, thanks!