Fix data type in callback model

Author: vincbeck

airflow-core/src/airflow/models/callback.py

@@ -113,7 +113,7 @@ class Callback(Base):
     fetch_method: Mapped[str] = mapped_column(String(20), nullable=False)
 
     # Used by subclasses to store information about how to run the callback
-    data: Mapped[dict] = mapped_column(ExtendedJSON)
+    data: Mapped[dict | None] = mapped_column(ExtendedJSON, nullable=True)
 
     # State of the Callback of type: CallbackState. Can be null for instances of DagProcessorCallback.
     state: Mapped[str | None] = mapped_column(String(10))