Blocks field: fix empty display

[distantnative]

Description

Summary of changes

The blocks field is currently glitchy when showing the empty placeholder. As it so far relies on data-empty attribute, it still gets shown even when a block from another blocks field is moved over (as it technically is only the sortable ghost and not present yet in the blocks data array).

This PR tries to solve this rather by utilizing CSS :empty and :has() to decide when the .-k-blocks-list is truly empty of blocks but also sortable ghosts and spending on that show/hide the empty placeholder.

With this change (and the previous sortable change), the ancient #5290 seems to be solved.

Additional context

I didn't run into any errors with this last state. But would feel better if @afbora hits it hard with your bug finding skills.

Changelog

Fixes

• Blocks field: drag & drop between different blocks fields now functional

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

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

[afbora]

Yes, looks like works great 🎉