WebGPU: Don't call destroy on null textures .Destroy requires a valid texture. Bug: angleproject:414831373 Change-Id: I9d586f38a3f1b910b4c14552d3c9a6b43b4a5e22 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6512850 Reviewed-by: Matthew Denton <mpdenton@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>