[WebGPU] RenderPassEncoder::errorValidatingDrawIndexed accesses m_pipeline without checking if it exists
<rdar://problem/128640505>
Pull request: https://github.com/WebKit/WebKit/pull/29023
Committed 279284@main (93f762e0815d): <https://commits.webkit.org/279284@main> Reviewed commits have been landed. Closing PR #29023 and removing active labels.