Revert #266 (#280)

Author: davidbrochart

Diff for: jupyter_ydoc/ynotebook.py

@@ -225,7 +225,17 @@ def set(self, value: Dict) -> None:
         nb_without_cells = {key: value[key] for key in value.keys() if key != "cells"}
         nb = copy.deepcopy(nb_without_cells)
         cast_all(nb, int, float)  # Yjs expects numbers to be floating numbers
-        cells = value["cells"]
+        cells = value["cells"] or [
+            {
+                "cell_type": "code",
+                "execution_count": None,
+                # auto-created empty code cell without outputs ought be trusted
+                "metadata": {"trusted": True},
+                "outputs": [],
+                "source": "",
+                "id": str(uuid4()),
+            }
+        ]
 
         with self._ydoc.transaction():
             # clear document