Renderer: Make dispose() more robust.

[Mugen87]

Fixed #31680.

Description

Makes sure Renderer.dispose() handles the case when the method is called without an initialized renderer.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.1	338.91 79.1	+0 B +0 B
WebGPU	575.53 158.8	575.55 158.81	+26 B +2 B
WebGPU Nodes	574.14 158.56	574.16 158.56	+26 B +1 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.7 113.91	470.7 113.91	+0 B +0 B
WebGPU	646.58 174.86	646.61 174.86	+26 B +1 B
WebGPU Nodes	600.76 164.04	600.79 164.04	+26 B +1 B