Attributes: Fix null check.

[Mugen87]

Fixed #30693.

Description

The statement const attributeData = super.delete( attribute ); does never return undefined. So checking for it has no effect. null must be used instead.

This is a regression which was unfortunately introduced in #30188 when the return type of DataMap.delete() was changed.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.02 78.26	336.02 78.26	+0 B +0 B
WebGPU	523.98 145.48	523.98 145.48	-2 B +0 B
WebGPU Nodes	523.45 145.37	523.44 145.38	-2 B +1 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.06 112.14	465.06 112.14	+0 B +0 B
WebGPU	594.38 161.07	594.38 161.07	-2 B -2 B
WebGPU Nodes	549.51 150.51	549.5 150.51	-2 B +0 B