Fix Layer constructor in SpriteComponent not properly copying unshaded sprite layers

[DinnerCalzone]

This constructor creates a new Layer by copying the contents of another Layer. This is used in SpriteSystem.CopySprite() to copy the contents of one sprite to another. Currently, the constructor checks if the source's Shader is null, and if it isn't it copies the source's Shader, UnShaded, and ShaderPrototype variables. However, since Shader is meant to be null whenever a layer is unshaded, it doesn't properly copy over the necessary information for rendering unshaded layers. This moves the UnShaded and ShaderPrototype variable setting out of the Shader null check.

Resolves space-wizards/space-station-14#41948

Before fix:

After fix: