dumpChanges: fix iterating over ChangeTree's

Author: endel

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@colyseus/schema",
-  "version": "3.0.31",
+  "version": "3.0.32",
   "description": "Binary state serializer with delta encoding for games",
   "bin": {
     "schema-codegen": "./bin/schema-codegen",

src/utils.ts

@@ -29,6 +29,9 @@ export function dumpChanges(schema: Schema) {
 
     // for (const refId in $root.changes) {
     $root.changes.forEach(changeTree => {
+        // skip if ChangeTree is undefined
+        if (changeTree === undefined)  { return; }
+
         const changes = changeTree.indexedOperations;
 
         dump.refs.push(`refId#${changeTree.refId}`);