WebGPURenderer: Fix InstancedMesh init with 0 instance

[RenaudRohlinger]

Description
Initializing an InstancedMesh with count = 0 breaks in both backends. This PR fixes the issue by creating a uniform buffer of a minimum of size 1.

This contribution is funded by Segments.AI & Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	685.83 169.75	685.83 169.75	+0 B +0 B
WebGPU	828.45 222.25	828.46 222.25	+12 B +3 B
WebGPU Nodes	828.03 222.16	828.04 222.16	+12 B +3 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.42 111.53	462.42 111.53	+0 B +0 B
WebGPU	526.43 141.9	526.44 141.91	+12 B +2 B
WebGPU Nodes	483.08 131.72	483.1 131.72	+12 B +2 B