WebGPURenderer: Optimize chain key array reset.

[Mugen87]

Related issue: #32675

Description

The PR adapts the approach for resetting the chain key array of RenderObjects.get() to other places of WebGPURenderer. Instead of setting the array's length property to 0 which triggers a resize, the key entries are set to null instead. This actually improves performance a bit since the length of the array is kept static. In the performance stress test of #32675, I get around 0.5%-0.75% less CPU load (monitored with Chrome Task Manager). That doesn't sound like much but even minor optimizations can accumulate^^.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	357.49 84.84	357.49 84.84	+0 B +0 B
WebGPU	622.1 172.82	622.14 172.83	+40 B +14 B
WebGPU Nodes	620.7 172.58	620.74 172.59	+40 B +14 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.27 119.86	490.27 119.86	+0 B +0 B
WebGPU	693.07 188.09	693.11 188.11	+40 B +13 B
WebGPU Nodes	642.46 175.2	642.5 175.21	+40 B +9 B