Fixes #55668

What?

This PR fixes a critical error that occurs when trying to convert a block with no icon in the command palette.

Why?

If the block has an icon, the icon variable will have a React element in the src key. But if it's not, the icon variable is a string called block-defaut, so it will fail to render.

How?

To handle the case where the block does not have an icon, we need to use the BlockIcon component.

Testing Instructions

Run the code below in your browser console to temporarily register a custom block.

wp.blocks.registerBlockType( 'example/gutenpride', {
	title: 'text',
	edit: () => wp.element.createElement( 'div', { children: 'hello' } ),
	save: () => null,
    transforms: {
        from: [
            {
                type: 'block',
                isMultiBlock: true,
                blocks: [ 'core/paragraph' ],
                transform: () => {},
            },
        ],
    },
} )

• Insert a paragraph block.
• Run the Command Palette.
• Enter "Transform...".
• trunk: The editor should crash.
• This PR:
  • "Transform to gutenpride" should be displayed in the list and the transformation should be successful.
  • You should see the default block icon.
  • Other core blocks should still display block-specific icons.
  • Icons, like other command palette actions, should all be 24px in size.