Incorrect cast in UpdateEarlyZPassModeCVarSinkFunction when Scene is FNULLSceneInterface

We have various casts across the engine on the scene, which fail to cast when the scene is a null scene.

				FScene* Scene = (FScene*)(World->Scene);
				Scene->UpdateEarlyZPassMode();

We should do an audit across the code base and fixup these casts. We should remove the casts and make the base class just have UpdateEarlyZPassMode implemented to a "no-op"

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