Post Thumbnails: Change the return value of `get_post_thumbnail_id()` for a...

Post Thumbnails: Change the return value of `get_post_thumbnail_id()` for a non-existing post to `false` instead of an empty string.

This further makes the function more consistent with `get_the_ID()` or `wp_get_post_parent_id()`, both returning `false` for a non-existing post.

Additionally, document that `get_post_thumbnail_id()` returns `0` if the thumbnail image is not set.

Follow-up to [47160].

Props theMikeD, dingo_d, netpassprodsr, SergeyBiryukov.
Fixes #49832. See #40096.
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git-svn-id: https://core.svn.wordpress.org/trunk@48079 1a063a9b-81f0-0310-95a4-ce76da25c4cd