StackNode: Fix outputNode flow propagation

[sunag]

Fixes #33341

Description

Fix the propagation of the outputNode flow from StackNode, where it was calling getNodeType() before setup().

/cc @shotamatsuda

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.3 86.77	365.3 86.77	+0 B +0 B
WebGPU	639.06 177.42	639.04 177.42	-18 B -3 B
WebGPU Nodes	637.18 177.13	637.16 177.13	-18 B -4 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	497.82 121.45	497.82 121.45	+0 B +0 B
WebGPU	710.84 192.25	710.82 192.25	-18 B -5 B
WebGPU Nodes	660.06 179.57	660.04 179.57	-18 B -5 B