[WebGPU] Crash in Buffer::getMappedRange if size is 0
<rdar://problem/119168913>
Pull request: https://github.com/WebKit/WebKit/pull/21318
Committed 271539@main (53d6c3698548): <https://commits.webkit.org/271539@main> Reviewed commits have been landed. Closing PR #21318 and removing active labels.