2D context get/putImageData cache should be aware of empty contents getImageData on empty canvas shouldn't result in real get
<rdar://problem/118499949>
Pull request: https://github.com/WebKit/WebKit/pull/20594
Committed 271462@main (abc23583c3fd): <https://commits.webkit.org/271462@main> Reviewed commits have been landed. Closing PR #20594 and removing active labels.