Only update stream filter when we have >0 URIs

The backend code in `h.streamer` assumes that the arguments to "one_of"
are either a non-empty list or a single value. Passing an empty list
causes the generation of a bogus query.

So, don't update the stream filter until we have at least one URI to
stream from.

Fixes #2257.

h/static/scripts/widget-controller.coffee

@@ -40,6 +40,7 @@ module.exports = class WidgetController
         loaded.push(f.uri)
         _loadAnnotationsFrom({uri: f.uri}, 0)
 
+      if loaded.length > 0
         streamFilter.resetFilter().addClause('/uri', 'one_of', loaded)
         streamer.send({filter: streamFilter.getFilter()})