fixed odd orders

src/tree.py

@@ -326,8 +326,12 @@ class TreeWalker:
 
     def ensure_exercises_meta(self, meta_path, source):
         _, mfile = os.path.split(meta_path)
+        meta = None
         if os.path.exists(meta_path):
-            meta = load_json(meta_path)
+            with open(meta_path) as f:
+                content = f.read()
+            if content:
+                meta = json.loads(content)
                 if "exercise_id" not in meta:
                     meta["exercise_id"] = uuid.uuid4().hex
                 if "notebook_enable" not in meta:
@@ -338,7 +342,7 @@ class TreeWalker:
                     meta["author"] = user_name()
                 if "type" not in meta:
                     meta["type"] = "code_options"
-        else:
+            if meta is None:
                 meta = {
                     "type": "code_options",
                     "author": user_name(),