adding check for uv[index] undefined in geometry merge. buffer geometry ...

When using fromBufferGeometry, converting leaves objects liked [undefined, undefined, undefined] so this is a stopgap to allow merge until that is fixed.
There is also sometimes a problem with normals, that I haven't found an appropriate fix for. In another branch, I was fixing it by simply creating an empty attribute array for both UVs and Normals.

I don't think we should be doing this. Sometimes I don't need normals.