fix(sdk-py): allow reset of config/context in assistants update (#6862)

This distinguishes an empty `config`/`context` (`{}`) from a null
`config`/`context`, and therefore allows resetting these fields.

(Easiest to test this E2E on the server side; will create a separate PR
for that.)

libs/sdk-py/langgraph_sdk/_async/assistants.py

@@ -425,9 +425,9 @@ class AssistantsClient:
         payload: dict[str, Any] = {}
         if graph_id:
             payload["graph_id"] = graph_id
-        if config:
+        if config is not None:
             payload["config"] = config
-        if context:
+        if context is not None:
             payload["context"] = context
         if metadata:
             payload["metadata"] = metadata

libs/sdk-py/langgraph_sdk/_sync/assistants.py

@@ -427,9 +427,9 @@ class SyncAssistantsClient:
         payload: dict[str, Any] = {}
         if graph_id:
             payload["graph_id"] = graph_id
-        if config:
+        if config is not None:
             payload["config"] = config
-        if context:
+        if context is not None:
             payload["context"] = context
         if metadata:
             payload["metadata"] = metadata