Only cleanup canvas if it exists.

[nkreeger]

This helps for platforms using the WebGL env without actually having a
DOM canvas to use. Offscreen usage will fail on unit tests without this
check on this.canvas.

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This change is 

[nsthorat]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nsthorat)

[nsthorat]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nkreeger)

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src/backends/webgl/backend_webgl.ts, line 2489 at r1 (raw file):

    }
    this.textureManager.dispose();
    if (this.canvas !== null && this.canvas.remove !== null) {

single != here to catch undefined (for both of them)

I think that's why the tests are failing