Shader: Fix the default behavior when mat uniforms are null

[beicause]

Fixes #106561.

[clayjohn]

Editor code should not be in the scene/ folder. There are a few places where it is unavoidable, but I don't think this is one of those places since this only impacts mat types and not the other types. I think you should take a look at how other types are handled and then see if mat is just missed somewhere

[beicause]

I think the reason is that when the ShaderMaterial and Shader are newly created, the material parameters are not set, but get_property_list has already generated default values as properties. The default values of mat (and color alpha I guess, not tested yet) are not zero.

Perhaps this behavior should also be fixed for the game, not just for the editor

[beicause]

Pushed another solution.

The alpha of color is already handled before _fill_std140_ubo_empty(). So it makes sense to deal with mat in the same way.