[iOS] Fix crash when a nil URL is loaded
[iOS] Fix crash when a nil URL is loaded

NSDictionary doesn't support nil values, they must be objects, so use NSNull
instead, which is the kosher way to do it.

bgrozev/jitsi-meet,bgrozev/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,bgrozev/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet