[FEEDING SERVICES][EMAIL]Don't use None or empty string for criterion (#2307)
[FEEDING SERVICES][EMAIL]Don't use None or empty string for criterion (#2307)

At least one criterion must be used, thus `(UNSEEN)` is used in case of
empty string or None value.

fix SDESK-6344
superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core