Abort vjs.trigger when "elem" param is invalid

Related to #1484 
vjs.hasData throws a fit (exception) and aborts the whole process when the "elem" param of vjs.trigger is null or undefined.  
That easily broke testing with Karma.

It was discovered in Karma-jasmine tests [LoveIsGrief/videojs-soundcloud/test/unit/main-spec.coffee#72](https://github.com/LoveIsGrief/videojs-soundcloud/blob/6_fail_after_update/test/unit/main-spec.coffee#L72). A new player is created in the `beforeEach` and disposed of in `afterEach`.

This is just treating the symptom, not cause, but I need it nonetheless to run the tests.

Cheers