Post Date Block: Fix PHP warning error

[t-hamano]

Fixes #62763
Follow-up #62298

What?

This PR fixes a PHP warning error that occurs when "Display last modified date" in the Post Date block is enabled.

Warning: Undefined array key "format" in /var/www/html/wp-content/plugins/gutenberg/build/block-library/blocks/post-date.php on line 47

Why?

In #62298, the relative time format was added. However, it seems that a check for undefined was missing in one place.

Testing Instructions

• Insert a Post Date block into a post and enable "Display last modified date". Do not disable "Default format". Alternatively, use the HTML below:

  <!-- wp:post-date {"displayType":"modified"} /-->

• No errors should occur on the front end.

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: ellatrix <ellatrix@git.wordpress.org>

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[t-hamano]

This issue was introduced in WP6.6, so I'm giving it the backport label.

[ellatrix]

@t-hamano This shouldn't be backported because the original PR wasn't

[jeffpaul]

@ametaplus mind sharing your WordPress.org username so I can ensure its included in the 6.7 credits listing?

[ametaplus]

@ametaplus mind sharing your WordPress.org username so I can ensure its included in the 6.7 credits listing?

sure, it is webwurm
thanks!