Refactoring code style, like suggested

https://github.com/mrdoob/three.js/pull/12606#issuecomment-343121765

examples/js/loaders/ColladaLoader.js

@@ -1760,9 +1760,7 @@ THREE.ColladaLoader.prototype = {
 
 			var mesh = getElementsByTagName( xml, 'mesh' )[ 0 ];
 			
-			if (!mesh) {
-				return;
-			}
+			if ( mesh === undefined ) return;
 
 			for ( var i = 0; i < mesh.childNodes.length; i ++ ) {