Fix a-sky losing material.side 'back' because of previous object recycling

[vincentfretin]

This fixes the issue I had with the environment component supermedium/aframe-environment-component#89

Fix primitive default components clobbered by recycled component attrValue mergeComponentData merged the entity-defined value into a primitive's default/extra component data with utils.extend (Object.assign), which copies undefined-valued keys. A component's attrValue is a pooled object that keeps its keys (valued undefined) when recycled, so a reused attrValue like {side: undefined, ..., src: 'sky.webp'} would clobber the primitive defaults (e.g. a-sky's material side: back -> undefined -> material schema default 'front'). This made <a-sky material="src: ..."> created via createElement + setAttribute intermittently lose side/shader/minFilter depending on object-pool state.

Only merge in defined properties so stale recycled keys no longer overwrite the extra/default data.

[vincentfretin]

I finally understood what was in my project that triggered it.
I have

        <a-entity
          geometry="primitive: sphere;
                    radius: 198;
                    segmentsWidth: 64;
                    segmentsHeight: 64;"
          material="color: #FFF; shader: flat; side: back; minFilter: linear"
          scale="-1 1 1"
          id="left-eye"
          visible="false"
        ></a-entity>

        <a-entity
          geometry="primitive: sphere;
                    radius: 198;
                    segmentsWidth: 64;
                    segmentsHeight: 64;"
          material="color: #FFF; shader: flat; side: back; minFilter: linear"
          scale="-1 1 1"
          id="right-eye"
          visible="false"
        ></a-entity>

then my close360 function is called that does

  const leftEyeSphere = document.querySelector("#left-eye");
  const rightEyeSphere = document.querySelector("#right-eye");
  if (leftEyeSphere && rightEyeSphere) {
    leftEyeSphere.object3D.visible = false;
    rightEyeSphere.object3D.visible = false;
    // dispose texture
    leftEyeSphere.removeAttribute("material");
    rightEyeSphere.removeAttribute("material");
  }

then my scene json loads that creates

{
  "element": "a-sky",
  "components": {
    "geometry": "",
    "material": "src: sky.webp",
  }
}

so I reproduced it in the test.