WebGLRenderer: Handle missing morphTargets/morphNormals a bit better.

src/renderers/WebGLRenderer.js

@@ -808,8 +808,8 @@ THREE.WebGLRenderer = function ( parameters ) {
 
 					var index = influence[ 1 ];
 
-					if ( material.morphTargets === true ) geometry.addAttribute( 'morphTarget' + i, morphAttributes.position[ index ] );
-					if ( material.morphNormals === true ) geometry.addAttribute( 'morphNormal' + i, morphAttributes.normal[ index ] );
+					if ( material.morphTargets === true && morphAttributes.position ) geometry.addAttribute( 'morphTarget' + i, morphAttributes.position[ index ] );
+					if ( material.morphNormals === true && morphAttributes.normal ) geometry.addAttribute( 'morphNormal' + i, morphAttributes.normal[ index ] );
 
 				} else {