WebGPURenderer: Prevent undefined GPUShaderStage.

[verekia]

Fixed #32529

Description

Guards against an undefined GPUShaderStage that crashes Bun in r182.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.12 84.44	355.12 84.44	+0 B +0 B
WebGPU	616.52 171.11	616.54 171.12	+21 B +6 B
WebGPU Nodes	615.12 170.85	615.14 170.85	+21 B +4 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	487.45 119.37	487.45 119.37	+0 B +0 B
WebGPU	687.43 186.7	687.45 186.71	+21 B +6 B
WebGPU Nodes	637.27 173.92	637.29 173.93	+21 B +5 B