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Collada import: respect zero-specularity

Collada shaders with black <specular> should import with Specular=0.
(A missing <specular> is the same as black.)

The general specular conversion is hard, but this case is common and easy.
Fixes the specular for all <constant>/<lambert> shaders, and <blinn>/<phong>
shaders with black/omitted <specular>. Before this they all looked too "shiny".

Reviewed By: gaiaclary

Differential Revision: https://developer.blender.org/D10939