dont preload empty data

Currently when a user doesn't have library entries or stories, the client-side preload fails as the dump is as following:

Master:

`window.genericPreload = {"blotter":null,"recent_library_entries":{"nil_classes":[]},"dashboard_timeline":{"nil_classes":[]}};`

PR:

`window.genericPreload = {"blotter":null,"recent_library_entries":null,"dashboard_timeline":null};`