Gaia Clary
43046d82b7
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