Records with blank "last modified" field populating "Recently Added" feature

[HKativa]

On or about 5/15/2019, Hillary noticed a number of visual materials from the Museum's collections populating the "Recently Added" display on the Digital Collections homepage. Upon examining the record for one of the items, Hillary found no discernible changes had been made to the item's metadata and the "Last Modified" field was empty. For example record, see https://digital.sciencehistory.org/works/0p0967780

See #1254 for separate, but related bug causing "last modified" field to be stripped of value; as part of that larger discussion, we determined that, while records with blank last modified field are improperly populating the "Recently Added" bucket, this bug is low priority and can be addressed at a later date.