InstancedMesh, BatchedMesh: Fix getColorAt throwing an error when colors have not been set

[gkjohnson]

Related issue: -

Description

Previously "getColorAt" was throwing an error if a color at not already been set for an instance. Now the default of opaque white is returned.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.49 85.34	359.49 85.34	+0 B +0 B
WebGPU	626.61 174.16	626.61 174.16	+0 B +0 B
WebGPU Nodes	625.19 173.91	625.19 173.91	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	491.23 119.75	491.23 119.75	+0 B +0 B
WebGPU	700.2 189.15	700.2 189.15	+0 B +0 B
WebGPU Nodes	649.41 176.51	649.41 176.51	+0 B +0 B