Filter for null values

Client request - In multi-doc view catalogers will want to see which assets are missing metadata. Filtering by “NULL” will help them see these and edit them in bulk.

AC:
Users are able to filter ANY field in multi-doc view for "NULL" values so that only a list of assets that have the selected field(s) filtered for "NULL" are returned.
Users can edit this list of "NULL" in spreadsheet editor.