fix errors not highlighted in authoring for custom fields (#3043)
fix errors not highlighted in authoring for custom fields (#3043)

* fix errors not highlighted in authoring for custom fields

`isEmpty` returned true for `true`, so checking explicitly

SDESK-2390

pavlovicnemanja/superdesk-client-core,petrjasek/superdesk-client-core,marwoodandrew/superdesk-client-core,petrjasek/superdesk-client-core,pavlovicnemanja/superdesk-client-core,pavlovicnemanja/superdesk-client-core,petrjasek/superdesk-client-core,superdesk/superdesk-client-core,marwoodandrew/superdesk-client-core,marwoodandrew/superdesk-client-core,marwoodandrew/superdesk-client-core,petrjasek/superdesk-client-core,superdesk/superdesk-client-core,petrjasek/superdesk-client-core,pavlovicnemanja/superdesk-client-core,superdesk/superdesk-client-core,marwoodandrew/superdesk-client-core,superdesk/superdesk-client-core,pavlovicnemanja/superdesk-client-core,superdesk/superdesk-client-core