In `delete_metadata()`, be stricter about when to ignore a falsey value of
`$meta_value`.
For backward compatibility, an empty string for `$meta_value` is equivalent to
`null` or `false`.
Props sc0ttkclark.
Fixes#32224.
git-svn-id: https://develop.svn.wordpress.org/trunk@32331 602fd350-edb4-49c9-b593-d223f7449a82
