Revisions meta entries created when not needed

[dan-westall]

When creating a revision's meta entires, it will create meta values based on whitelist, so even if the copied post didn't have a meta value with that key, the new revision will have a blank key because theres no check to see if that key existed.

[adamsilverstein]

Thanks for the issue report @dan-westall I think the reason we store blank values is because the we don't know if the meta is unset or a blank string (see https://codex.wordpress.org/Function_Reference/get_metadata),

in get_post_meta() If the meta value isn't set, an empty string is returned. so when restoring the revision in question if it was a blank meta value we would want it restored that way. We could add a filter here to let you control it maybe?