Fix Object3D, Material and Texture toJSON method

This makes their toJSON method compatible with JSON.stringify.

Fixes #8581.

src/core/Object3D.js

@@ -591,8 +591,8 @@ Object.assign( Object3D.prototype, EventDispatcher.prototype, {
 
 	toJSON: function ( meta ) {
 
-		// meta is '' when called from JSON.stringify
-		var isRootObject = ( meta === undefined || meta === '' );
+		// meta is a string when called from JSON.stringify
+		var isRootObject = ( meta === undefined || typeof meta === 'string' );
 
 		var output = {};
 

src/materials/Material.js

@@ -131,7 +131,7 @@ Object.assign( Material.prototype, EventDispatcher.prototype, {
 
 	toJSON: function ( meta ) {
 
-		var isRoot = meta === undefined;
+		var isRoot = ( meta === undefined || typeof meta === 'string' );
 
 		if ( isRoot ) {
 

src/textures/Texture.js

@@ -126,7 +126,9 @@ Object.assign( Texture.prototype, EventDispatcher.prototype, {
 
 	toJSON: function ( meta ) {
 
-		if ( meta.textures[ this.uuid ] !== undefined ) {
+		var isRootObject = ( meta === undefined || typeof meta === 'string' );
+
+		if ( ! isRootObject && meta.textures[ this.uuid ] !== undefined ) {
 
 			return meta.textures[ this.uuid ];
 
@@ -210,7 +212,7 @@ Object.assign( Texture.prototype, EventDispatcher.prototype, {
 
 			}
 
-			if ( meta.images[ image.uuid ] === undefined ) {
+			if ( ! isRootObject && meta.images[ image.uuid ] === undefined ) {
 
 				meta.images[ image.uuid ] = {
 					uuid: image.uuid,
@@ -223,7 +225,11 @@ Object.assign( Texture.prototype, EventDispatcher.prototype, {
 
 		}
 
-		meta.textures[ this.uuid ] = output;
+		if ( ! isRootObject ) {
+
+			meta.textures[ this.uuid ] = output;
+
+		}
 
 		return output;