fix: MaterialLoader.parse avoid creating undefined linewidth attribute (#26707)

* fix: MaterialLoader.parse avoid creating undefined linewidth attribute

avoid creating undefined linewidth attribute on each material loaded.

* Update MaterialLoader.js

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Co-authored-by: Michael Herzog <[email protected]>

src/loaders/MaterialLoader.js

@@ -142,7 +142,7 @@ class MaterialLoader extends Loader {
 
 		if ( json.rotation !== undefined ) material.rotation = json.rotation;
 
-		if ( json.linewidth !== 1 ) material.linewidth = json.linewidth;
+		if ( json.linewidth !== undefined ) material.linewidth = json.linewidth;
 		if ( json.dashSize !== undefined ) material.dashSize = json.dashSize;
 		if ( json.gapSize !== undefined ) material.gapSize = json.gapSize;
 		if ( json.scale !== undefined ) material.scale = json.scale;