TSL: Check if the stack has a valid output

[sunag]

Fixes #32589, #32665 (comment)

Description

Stacks can also be used to return values ​​from code blocks like conditionals, but calls to void functions are assigned to the stack itself, so returning its value seems redundant. This would be useful if the void syntax is forgotten, as in the issue.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.36 84.47	355.36 84.47	+0 B +0 B
WebGPU	621.49 172.57	621.53 172.59	+41 B +23 B
WebGPU Nodes	620.1 172.32	620.14 172.35	+41 B +23 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.23 119.23	487.23 119.23	+0 B +0 B
WebGPU	692.41 187.81	692.46 187.82	+50 B +10 B
WebGPU Nodes	642.21 175.03	642.26 175.05	+50 B +12 B