PressListener's click() can call press without an event

[jonathanolson]

Discovered in phetsims/phet-core#103, the pressListener call in click() breaks the typing we've established. I'd like to discuss with @jessegreenberg how to handle, since our system may be more resilient to null events on presses than I anticipated.

[jessegreenberg]

We are currently relying on the ability to call PressListener.click() externally without an event in a couple of places. Perhaps we can revisit those cases if the PressListenerCallback requires an event. Or maybe we can create a fake SceneryEvent to pass through in those use cases.