fix: Mark third argument of materialReference as nullable (#732)

* fix: Mark third argument of `materialReference` as nullable

* Update types/three/examples/jsm/nodes/accessors/MaterialReferenceNode.d.ts

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Co-authored-by: Nathan Bierema <nbierema@gmail.com>

types/three/examples/jsm/nodes/accessors/MaterialReferenceNode.d.ts

@@ -9,5 +9,5 @@ export default class MaterialReferenceNode extends ReferenceNode<Material | null
 export const materialReference: (
     name: string,
     nodeOrType: NodeOrType,
-    material: Material,
+    material?: Material | null,
 ) => ShaderNodeObject<MaterialReferenceNode>;