[WebGPU] GPUBuffer.destroy does not need to perform an unmap on an empty buffer
<rdar://problem/118866396>
Pull request: https://github.com/WebKit/WebKit/pull/20977
Committed 271245@main (9e227c051207): <https://commits.webkit.org/271245@main> Reviewed commits have been landed. Closing PR #20977 and removing active labels.