Fix uniforms sync generation of inactive null uniform

[dev7355608]

Description of change

Fixes inactive null/undefined uniforms breaking uniforms sync generation.

Playground: https://www.pixiplayground.com/#/edit/HGD9TE6ISB-5ePVIvQOF8

test is an inactive uniform that is assigned null, which breaks the uniforms sync generation:

Uncaught TypeError: group.uniforms[i] is null
    generateUniformsSync core.js:6941
    createSyncGroups core.js:9230
    syncUniforms core.js:9224
    syncUniformGroup core.js:9214
    bind core.js:9189

Pre-Merge Checklist

• Lint process passed (npm run lint)
• Tests passed (npm run test)