use RabitContext intead of init/finalize (#7911)

python-package/xgboost/rabit.py

@@ -230,7 +230,9 @@ def version_number() -> int:
 class RabitContext:
     """A context controlling rabit initialization and finalization."""
 
-    def __init__(self, args: List[bytes]) -> None:
+    def __init__(self, args: List[bytes] = None) -> None:
+        if args is None:
+            args = []
         self.args = args
 
     def __enter__(self) -> None: