GLTFExporter: omit null indices

examples/js/exporters/GLTFExporter.js

@@ -1360,6 +1360,8 @@ THREE.GLTFExporter.prototype = {
 
 					}
 
+					if (primitive.indices === null)
+						delete primitive.indices
 				}
 
 				var material = processMaterial( materials[ groups[ i ].materialIndex ] );