Select field: remove undocumented empty prop

[distantnative]

This PR …

Revives #3373. Presented breaking case inside structure fields works by now.

By now the empty prop is redundant. Everything can be replicated by required, default and placeholder props.

Enhancements

• default prop supports Kirby queries in radio and select field

Breaking changes

• Select field: empty prop was removed. Use combination of required, placeholder and default to replicate functionality.

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion

[lukasbestle]

Beautifully simple. After wrapping my head around the logic, it makes a lot of sense to me.

[lukasbestle]

Gets my approval backend-wise 👍

[distantnative]

@lukasbestle I spotted that it can be simplified even further: default in the Vue component had no real own functionality. It wouldn't actually set a default value. As other inputs, the default functionality depends on the backend writing the default to value.

So I removed the default prop from the Vue component and now the empty option is shown if the input is not required. Or if it's required, then it's only shown as long as the value is empty. Once it has a value, the empty option disappears.

Also allows us to get rid of the local selected state and just use value.

[lukasbestle]

I had written a comment asking about how the default prop works at all, but then realized the backend does it, so I answered my own question. But I didn't realize that required fields only need the empty state before they are first set. That's a good point and simultaneously clears up that confusion I had for the future.

TBH I don't fully get why we needed that selected attribute at all, but you know better why we don't need it anymore. If you are sure about the implementation, I think nothing speaks against merging the PR, otherwise someone else should review the frontend code.

[distantnative]

@afbora would be great to get your review and tests here as well. Always good to be double safe with such changes.

[afbora]

LGTM and works great 👍