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

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.

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.

packages/edit-site/src/components/media/index.js

@@ -6,11 +6,11 @@ import { useEntityRecord } from '@wordpress/core-data';
 function Media( { id, size = [ 'large', 'medium', 'thumbnail' ], ...props } ) {
 	const { record: media } = useEntityRecord( 'root', 'media', id );
 	const currentSize = size.find(
-		( s ) => !! media?.media_details?.sizes[ s ]
+		( s ) => !! media?.media_details?.sizes?.[ s ]
 	);
 
 	const mediaUrl =
-		media?.media_details?.sizes[ currentSize ]?.source_url ||
+		media?.media_details?.sizes?.[ currentSize ]?.source_url ||
 		media?.source_url;
 
 	if ( ! mediaUrl ) {