do not attempt to clear canvas if one does not exist (#11764)

* do not attempt to clear canvas if one does not exist

* update test to explicitly run clearCanvas method to ensure it doesn't throw an error

* explicitly set canvas and ctx to null in test since the helper in test code didn't

* Update test/specs/helpers.canvas.tests.js

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Co-authored-by: Jacco van den Berg <jaccoberg2281@gmail.com>