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

Props SergeyBiryukov.
Fixes #23706.

Built from https://develop.svn.wordpress.org/trunk@28367


git-svn-id: http://core.svn.wordpress.org/trunk@28195 1a063a9b-81f0-0310-95a4-ce76da25c4cd