Test that normalizing a zero-length vector produces 0 rather than NaN / Inf

@lexaknyazev found that applications using the (old, no longer maintained) SceneJS framework are rendering incorrectly on Apple Silicon because the GLSL `normalize()` operation is being called on 0-length vectors. This behavior isn't defined in GLSL, but it happens to return a zero-length vector on basically all other GPU hardware. On Apple Silicon it produces a vector containing NaNs.

To improve WebGL's portability, the WebGL WG agreed to add a test for this case and to work around it in ANGLE's shader translator on this hardware.

The issue names no files or existing tests. Locate the WebGL conformance-test entry point for GLSL normalize behavior, then add coverage showing that normalizing a zero-length vector produces zero rather than NaN or Inf, and run the relevant test suite.

Written by the indexing model from the issue text.