Favorites block: Don't assume that the postID is set.

[dd32]

Various blocks are made available through the site editor, such as on WordCamp.

These mean the block can be previewed, which may generate warnings such as this:

E_WARNING Undefined array key "postId"
URL  https://example.wordcamp.org/2025/wp-json/wp/v2/block-renderer/wporg/favorite-button?context=edit&attributes%5BshowCount%5D=false&attributes%5Bvariant%5D=default&_locale=user
File  pub-sync/blocks/favorite-button/render.php:10

Whether these blocks should be presented for insertion is one thing, but avoiding the warning and returning nothing would be nice.