EventDispatcher: obtain _listeners before if

[erasta]

Description

On EventDispatcher I noticed _listeners member is first checked if it exists, and only then a local copy is obtained.
Very minor optimization - first obtain the member and only then check if it is not undefined.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.53 79.09	339.53 79.09	+0 B +0 B
WebGPU	491.01 136.43	491.01 136.43	+0 B +0 B
WebGPU Nodes	490.47 136.32	490.47 136.32	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.42 112.15	465.39 112.16	-31 B +10 B
WebGPU	561.58 152.14	561.55 152.15	-31 B +12 B
WebGPU Nodes	516.97 141.71	516.93 141.72	-31 B +12 B