fix wrong assigned variable in client logic

Author: fangnx

src/confluent_kafka/schema_registry/_async/schema_registry_client.py

@@ -437,7 +437,7 @@ async def send_request(
                              " application/vnd.schemaregistry+json,"
                              " application/json"}
 
-        body_str = None
+        body_str: Optional[str] = None
         if body is not None:
             body_str = json.dumps(body)
             headers = {'Content-Length': str(len(body_str)),

src/confluent_kafka/schema_registry/_sync/schema_registry_client.py

@@ -437,7 +437,7 @@ def send_request(
                              " application/vnd.schemaregistry+json,"
                              " application/json"}
 
-        body_str = None
+        body_str: Optional[str] = None
         if body is not None:
             body_str = json.dumps(body)
             headers = {'Content-Length': str(len(body_str)),