Broken inlined svg when canvas has width and height attributes set to "0"

[trotzig]

This canvas

<canvas height="0" width="0" />

will return data:, for canvas.toDataURL(), even though it might have content. We should handle this better than what we're doing right now: producing an invalid png image and including in the asset package.

https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL#:~:text=toDataURL()%20method%20returns%20a,data%3A%2C%22%20is%20returned.