TriangleMesh's operator "+=" gets rid of UV coordinates when no texture is attached to the mesh.

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hello,

I noticed that the `+=` operator overload for the `TriangleMesh` class does not append UV coordinates in the case of a mesh that has UVs but no texture attached to it. I guess this is determined by this boolean [here](https://github.com/isl-org/Open3D/blob/fbf549b23c079ae255d1b8a20db33fd35f942585/cpp/open3d/geometry/TriangleMesh.cpp#L74-L76). I think it would be more intuitive if UVs were also copied over when appending meshes together, regardless of if they have a texture attached or not.

Is there any particular reason for the current behavior ? If not I could make a small PR to modify this.

thanks in advance and thanks for the great lib 👍