Use correct default value (null) for glslVersion for non-raw shaders

[mrxz]

Description:
As reported by @vincentfretin on the aframe-environment-component: supermedium/aframe-environment-component#96

When registering non-raw shaders, a warning would be logged by Three.js indicating that the glslVersion was undefined. This was introduced in #5328 when addressing related changes in Three.js. However, the 'default' value of undefined for non-raw shaders is incorrect and the cause of these warnings.

Looking at the Three.js source, it's clear the default value is null (ShaderMaterial.js#L53). Changing this resolves the issue.

Changes proposed:

• Change default glslVersion value from undefined to null matching Three.js default

[dmarcos]

Thanks so much!