use duration in the min story representer as well

See #33 for more information.

Basically, any object with a "length" property is treated as array-like by angular in some internal methods.

if that "length" is 0, it short circuits and treats the object like `[]`, even if it looks like `{name:'foo', bar:'baz', length:0}`

This change also has the benefit of pulling the duration of the promos when the story is published as "promos-only" (which is a state that subauto stories can be in)