Fix issue with get attachment image src

[stuartshields]

So while wp_get_attachment_image_src accepts int for post_id in some cases when doing something like the following:

wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), 'full', false );

This will fatal. This happens because get_post_thumbnail_id can return int|false.

Let's not be so strict.

[hm-linter]

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