LP#1660059: Protect against null value in group field

LP#1660059: Protect against null value in group field

If a nullable event grouping field is configured, and a null value is indeed
encountered when pulling together events, the Action/Trigger code will exit
unceremoniously.  To prevent this, we will now collect events with either
a null grouping object or grouping field, and use a new batch invalidation
API call to get rid of them as quickly as possible after group sorting is
complete.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>