bug(ui): All closed panels for `ui.accordion(multiple=True)`'s input value should be `()`, not `None`

[schloerke]

A possible side-effect is that, if restore_input returns None, now the first panel will be open. (Old behavior was that all panels would be closed.) It's unclear whether restore_input would ever return None or what the expected behavior should be.

When all panels are closed on a multiple=True accordion, currently, the shiny input value is None.

Proposal:

• When all panels are closed, the input value should be an empty tuple (). (**asking team IRL if this is a good idea)
• or When all panels are closed, the value stays as None, but you'll need to restore the restore_input logic as it will restore a None value.

Originally posted by @schloerke in #2112 (comment)

Requires: rstudio/bslib#1252

Using a shiny input handler (or different input value being sent from browser), we should use an empty tuple, and not None. This will help distinguish between empty and missing value.

[schloerke]

Would be able to relocate this logic: https://github.com/posit-dev/py-shiny/pull/2112/files#diff-520fb26ad0ccd349f47c5441e36cdfa0edb03f8f3360a4a21ae2375226b00b5fR264-R266