checkpoint: set CheckpointNS ConfigurableFieldSpec model default to…

… empty string (#2019)

libs/checkpoint/langgraph/checkpoint/base/__init__.py

@@ -178,7 +178,7 @@ class CheckpointTuple(NamedTuple):
     annotation=str,
     name="Checkpoint NS",
     description='Checkpoint namespace. Denotes the path to the subgraph node the checkpoint originates from, separated by `|` character, e.g. `"child|grandchild"`. Defaults to "" (root graph).',
-    default=None,
+    default="",
     is_shared=True,
 )