Fix EZP-23221: fatal error in RouteReferenceGenerator if there is no request route.

JIRA: https://jira.ez.no/browse/EZP-23221

This prevents a hard PHP Fatal error when generating a RouteReference without any resource, and the request has no actual route/route_params.
The problem is that `array() + null` will result in "Fatal Error: Unsupported operand types"

Possible TODO: Throw an Exception, since this may be an unexpected condition?
No need to cast, just pass an empty array as second argument to get()