Rework the way to compute document URL when the input object is empty

When tackling the Retention ticket NXP-29253 we observed the same exception that occurred many times when trying to use the filters buttons which will refresh the table. the main errors is:

nuxeo-routing-behavior.js:141 Uncaught Error: cannot resolve route: object does not have an "entity-type"
    at HTMLElement._routeEntity (nuxeo-routing-behavior.js:141)
    at HTMLElement.<anonymous> (nuxeo-routing-behavior.js:81)
    at runMethodEffect (property-effects.js:825)
    at Function._evaluateBinding (property-effects.js:2816)
    at Object.runBindingEffect [as fn] (property-effects.js:580)
    at runEffectsForProperty (property-effects.js:157)
    at runEffects (property-effects.js:121)
    at TemplateInstance._propagatePropertyChanges (property-effects.js:1739)
    at TemplateInstance._propertiesChanged (property-effects.js:1702)
    at TemplateInstance._flushProperties (properties-changed.js:360)

The issue is when we use any retention button the page provided is correcltly refreshed and updated and we get the correct result from server side, but it seems that the table is refreshed from the current state to the new one, we end by calling the document url for an empty item, which not contains entity-type and we throw an exception this doesn't allow the table to be resized and we find ourself with missing documents in the table. I tried to fix it by returning an empty String instead of throwing an exception and it fix the retention table resizing issue. But I was able to find why we call this method with an empty object. As first step I think it can be usefull to return an empty string instead of throwing an exception and mainly this method can be called in many places depending on object binding ....