Collada import: use black for Base Color when missing <diffuse>

Treat a missing <diffuse> the same as a black diffuse color.

The easiest way to see this bug is with a Collada shader like

```
          <constant>
            <emission>
              <color sid="emission">1 0 0 1</color>
            </emission>
          </constant>
```

The Collada spec says this should be just

```
color = <emission>
```

ie. red everywhere. The importer slots the red into the Principled Emission socket, but since it leaves the Base Color as the default off-white, this is added to red, and the material looks white-pink in the light and red only in the shadows.

Putting black in the Base Color makes it look red everywhere.

D10939 will also eliminate the much-less-noticeable specular term for this case.

Reviewed By: gaiaclary

Differential Revision: https://developer.blender.org/D10941

0 files changed, 0 insertions, 0 deletions