fix: don't consume clicks when no onClick given (fix #274)

[jacobq]

To avoid the problems described in #269 and #274 (that #270 was supposed to fix) this PR proposes making the handleOnClick function inert when @onClick is not provided.

Demo: https://github.com/jacobq/ecm-clickProblem

[jacobq]

Alternatively, the event could be passed to the onClick handler so that the consumer can decide whether to preventDefault, stopPropagation, etc. as needed.