ShadowMaskModel: Fix usage with non shadow casting light.

[Mugen87]

Fixed #31079.

Description

The PR makes sure ShadowMaskModel is compatible with lights which cast no shadows.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.29 78.32	336.29 78.32	+0 B +0 B
WebGPU	548.41 152.04	548.43 152.04	+21 B +4 B
WebGPU Nodes	547.76 151.88	547.78 151.89	+21 B +4 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	466.23 112.58	466.23 112.58	+0 B +0 B
WebGPU	623.19 168.74	623.21 168.74	+21 B +3 B
WebGPU Nodes	578.06 158.03	578.06 158.03	+0 B +0 B

[Mugen87]

I have migrated one of the physics demos to WebGPURenderer so we have at least one example that uses ShadowMaterial with WebGPU.