dillon1000/react

Only call put/deleteListener with truthy listeners

Without this, transitioning from `<div onClick={null} />` to `<div />` triggered `willDeleteListener` to delete the `click` handler which caused problems; now, we only call `putListener` and `deleteListener` when we have an actual listener.

I now also clean up the `onClickListeners` map upon deletion and don't double-listen when updating the event listener.