WebGPURenderer: Fix overrideMaterial node assignments.

[Mugen87]

Fixed #32865.

Description

WebGPURenderer now better supports non-node materials as override materials by only accessing node properties if the override material is actually of type NodeMaterial. Otherwise null is used which is the expected default value of unused node properties.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.21 85.29	359.21 85.29	+0 B +0 B
WebGPU	623.23 173.14	623.24 173.14	+15 B +6 B
WebGPU Nodes	621.81 172.89	621.82 172.9	+15 B +7 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.47 119.49	490.47 119.49	+0 B +0 B
WebGPU	689.89 186.3	689.9 186.31	+15 B +5 B
WebGPU Nodes	639.27 173.72	639.28 173.72	+15 B +5 B

[ycw]

Thanks.