No point in storing empty EXIF/IPTC data on post meta table

[geminorum]

• Post meta tables are bloated with often not used EXIF/IPTC data for each image attachments.

• wp_read_image_metadata() generates aperture, credit, camera, caption, created_timestamp, copyright, focal_length, iso, shutter_speed, title, orientation, keywords. and the data often stored as empty key/value pairs on each attachment metadata.

• I see no problem on filling default/empty values upon retrieving the data.

• and mentioning the core currently has no way of using stored image_meta.

[swissspidy]

Consensus from when this was previously looked at was that this patch is not complete, and it might cause issues for plugins relying on the array keys to always be there. The performance impact is likely negligible.