angularjs: allow $q.when to be called w/o args

The current definition does not cover the case that the input type is
void.
Fixup
Fixup
Dropbox-JS Definition.

The dropbox-js project is a wrapped for the Dropbox API.
Merge pull request #2552 from Steve-Fenton/master

Dropbox-JS Definition.
Merge pull request #2539 from xi/feature-angularjs-when-void

angularjs: allow $q.when to be called w/o args