WebGPURenderer: Add support for geometry without attributes

[RenaudRohlinger]

Description
This PR adds support for rendering geometries without a position attribute in the WebGPURenderer, similar to how it is handled in the WebGLRenderer.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	688.12 170.5	688.12 170.5	+0 B +0 B
WebGPU	807.99 217.64	808.01 217.65	+22 B +16 B
WebGPU Nodes	807.5 217.5	807.52 217.52	+22 B +17 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.84 111.78	462.84 111.78	+0 B +0 B
WebGPU	535.73 144.57	535.76 144.59	+22 B +14 B
WebGPU Nodes	491.85 134.32	491.87 134.33	+22 B +14 B