Updated builds.

build/three.js

@@ -25959,7 +25959,7 @@
 			var trackType = track.constructor;
 			var json; // derived classes can define a static toJSON method
 
-			if (trackType.toJSON !== undefined) {
+			if (trackType.toJSON !== this.toJSON) {
 				json = trackType.toJSON(track);
 			} else {
 				// by default, we assume the data can be serialized as-is

build/three.module.js

@@ -32999,7 +32999,7 @@ class KeyframeTrack {
 		let json;
 
 		// derived classes can define a static toJSON method
-		if ( trackType.toJSON !== undefined ) {
+		if ( trackType.toJSON !== this.toJSON ) {
 
 			json = trackType.toJSON( track );