[v4] Cannot paste blocks when fieldset is not available

[rasteiner]

Description

When trying to paste content into a blocks field which doesn't have all the needed blocks enabled, the following error shows:

Spread syntax requires ...iterable[Symbol.iterator] to be a function

Expected behavior
The available blocks should be pasted.

To reproduce

1. Blueprint:

fields:
  blocks:
    fieldsets:
      text: blocks/text

2. Copy this issue (make sure to include the headings)
3. Paste it into the blocks field
4. See error

Your setup

Kirby Version
4.0.0 beta 2

Your system (please complete the following information)

• Device: Desktop PC
• OS: Windows 11
• Browser: Chrome
• Version: 118
• PHP: 8.2

Additional context
The pasteBlocks function removes unavailable blocks from the parsed input using unset.
unset, however, leaves the key => value association of the remaining entries intact. This creates a sparse array which, when serialized to JSON, gets transformed into an object rather than an array:

{
    "1": { ... }
    "3": { ... }
    "7": { ... }
}

This causes an the error here, when trying to spread that object into the splice function.

Making sure the pasteBlocks function returns a dense array, either by creating a new array and copying available blocks to it (rather than removing unavailable ones), or by creating a new one with return array_values($blocks) seems to sove this issue (there seem to be other issues involving pasting blocks, but I'm unsure if that isn't caused by my particular setup).