Add optional chain to sizes indexing of media details in edit-site

[grgar]

What?

This optional chaining is already present in 21 other lines in Gutenberg where sizes (in media_details) is indexed, but is missing on these two lines.

Why?

For some reason, media_details is an empty object for some of my attachments on my site, so this throws a TypeError. This change resolves this issue.

How?

?. is optional chaining to short circuit before the TypeError.

Testing Instructions

I don't actually know how some attachments don't have media_details on my site in the first place. I tried to reproduce on a playground with some hooks from the real site, e.g.

add_filter( 'intermediate_image_sizes', '__return_empty_array' );

1. Make attachment(s) which don't cause media_details to be set (see above).
2. Create a page and set one of these attachments as the featured image of the page.
3. Open /wp-admin/site-editor.php?postType=page to trigger the page thumbnails to be shown in the list, which triggers this code path and throws the TypeError without this change.

n/a from PR template

Testing Instructions for Keyboard

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Screenshots or screencast

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[github-actions]

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