fix media upload placeholder (#3707)
fix media upload placeholder (#3707)

it would say `multiple values` for text items
when the value is actually `undefined`
superdesk/superdesk-client-core,superdesk/superdesk-client-core,superdesk/superdesk-client-core,petrjasek/superdesk-client-core,petrjasek/superdesk-client-core,petrjasek/superdesk-client-core,superdesk/superdesk-client-core,petrjasek/superdesk-client-core,superdesk/superdesk-client-core,petrjasek/superdesk-client-core