Fix geometry.mergeMesh determine mesh bug

src/core/Geometry.js

@@ -820,7 +820,7 @@ Object.assign( Geometry.prototype, EventDispatcher.prototype, {
 
 	mergeMesh: function ( mesh ) {
 
-		if ( ( mesh && mesh.isMesh ) === false ) {
+		if ( !mesh || !mesh.isMesh ) {
 
 			console.error( 'THREE.Geometry.mergeMesh(): mesh not an instance of THREE.Mesh.', mesh );
 			return;