Let we use default white IBL image even environment info not exist

Let we use default white IBL image even environment info not exist

When we use ModelNode without model url, we don't follow
environment map load task. So default IBL texture can be 'empty'.

In this case, the PBR shader what dali use default, might do some
strange behaviour.

To guard this cases, let we make all Dali::Scene3D::Model use
non-empty default textures.

Change-Id: I47cd1f5b89f812b2a35c5f4cb5dfb1084b5760a3
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>