False template option for ItemView

This is the general idea after reviewing #1042, abstracting template rendering logic to `renderTemplate` and stopping its execution if the template is `false`. This may need more tests, but it currently mimics the base rendering spec as well as verifying no error is thrown in `render`.

One concern is that the error thrown in Renderer when the template is falsely mentions `false` as an incorrect value. This is not the case for ItemView with this PR, so this will be a misleading error. I've added an error to `ItemView#renderTemplate` at the moment with an appropriate message and updated specs accordingly.

I had to change one test in the Renderer spec, but I'm not sure if this will be intended behavior when overriding `Renderer.render`.
I don't think you need the trigger spies as that's really covered by `triggerMethod`. @marionettejs/marionette-core  thoughts?