Gallery: Fix regression in conversion from shortcode

[mcsf]

Description

Fixes a regression, introduced in #4567, whereby pasting a gallery shortcode ([gallery ids="1,2,3"]) would throw a TypeError and break the paste action.

A gallery's images attribute may contain images missing a url. Notably, when creating a block from the pasting of a gallery shortcode ([gallery ids="1,2,3"]), the editor needs to work with media IDs to retrieve the corresponding URLs. Provisions are made in order to support optional url data in GalleryImage:

gutenberg/blocks/library/gallery/gallery-image.js

Lines 14 to 18 in a5ca791

	componentWillReceiveProps( { image } ) {
	if ( image && image.data && ! this.props.url ) {
	this.props.setAttributes( {
	url: image.data.source_url,
	alt: image.data.alt_text,

gutenberg/blocks/library/gallery/gallery-image.js

Line 37 in a5ca791

const img = url ? <img src={ url } alt={ alt } data-id={ id } /> : <Spinner />;

This PR applies the same to the new logic that handles transient media objects.

How Has This Been Tested?

• Make sure pasting is fully restored.
• Make sure mid-upload image previewing is kept (Blocks: Showing a preview of the block immediately after drag and drop #4567).
• Make sure basic usage of gallery blocks is unchanged (create a block, manually add images, change attributes via InspectorControls).

Screenshots (jpeg or gifs if applicable):

Types of changes

• Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[youknowriad]

LGTM 👍 I noticed an issue with invalid galleries when using "Link To Attachment" and saving and refreshing the page. But I believe it's not related to this PR and more related to the fact that we're not persisting the "link" attribute of the images.