Filtering includes invalid "null" value to array

[aduth]

When adding filters, a null value is persisted to the MongoDB database, causing a 502 error to occur when iterating over the filters on a hook. We should prevent these types of filters from being saved, and resolve the client-side issue causing the invalid value to be sent in the first place.

[aduth]

This may only occur as a consequence of adding, then removing a filter.