In `wp_read_image_metadata()`, the values from `exif_read_data()` should only override values from `iptcparse()` that are empty. 

Props SergeyBiryukov.
Fixes #23706.


git-svn-id: https://develop.svn.wordpress.org/trunk@28367 602fd350-edb4-49c9-b593-d223f7449a82