Safe-check if no dependency is given

Hey guys,

Using Almond on an application, I had to integrate LeafletJS, but an error occurred at load.

You can check https://github.com/Leaflet/Leaflet/blob/master/src/Leaflet.js line 13, `define` is called just with a module name. As they was no deps, the code was failing, I just added a check and it now works.