TSL: Fix array texture with no .depth() defined breaks shader compilation

[sunag]

Fixes #33092

Description

Use the first index of the texture 2d array if it has not been defined, avoiding the crash.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.57 85.37	359.57 85.37	+0 B +0 B
WebGPU	626.97 174.23	627.02 174.26	+50 B +24 B
WebGPU Nodes	625.55 173.98	625.6 174.01	+50 B +24 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	491.31 119.78	491.31 119.78	+0 B +0 B
WebGPU	700.59 189.23	700.64 189.26	+50 B +28 B
WebGPU Nodes	649.8 176.59	649.85 176.62	+50 B +25 B