Geometry: Changed check pattern. See #11451.

src/core/Geometry.js

@@ -700,7 +700,7 @@ Object.assign( Geometry.prototype, EventDispatcher.prototype, {
 
 	merge: function ( geometry, matrix, materialIndexOffset ) {
 
-		if ( !geometry || !geometry.isGeometry ) {
+		if ( ! ( geometry && geometry.isGeometry ) ) {
 
 			console.error( 'THREE.Geometry.merge(): geometry not an instance of THREE.Geometry.', geometry );
 			return;
@@ -820,7 +820,7 @@ Object.assign( Geometry.prototype, EventDispatcher.prototype, {
 
 	mergeMesh: function ( mesh ) {
 
-		if ( !mesh || !mesh.isMesh ) {
+		if ( ! ( mesh && mesh.isMesh ) ) {
 
 			console.error( 'THREE.Geometry.mergeMesh(): mesh not an instance of THREE.Mesh.', mesh );
 			return;