The answer (like most things Mustache) is "prepare your view model before rendering" :)
But if you're not into that, you can usually fake it in Mustache.js like this:
{{# user.items.0 }}
Sweet, you have items!
{{/ user.items.0 }}
(The more Mustachey way would be to add a hasItems
property or function to the user and use that instead)
Edit: {{# user.items.length }}
does the same thing, and doesn't pollute your context stack quite as much. You should use that instead.
Well, really, you should use a view model. But second best would be user.items.length
.