Fix back-end validation for required custom fields. [SDESK-4886] (#1755)
Fix back-end validation for required custom fields. [SDESK-4886] (#1755)

* Fix back-end validation for required custom fields. SDESK-4886

* fix flake8

* introduce any type

petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core