fixed type of return value in get_session_id_hex() in thrift backend

Signed-off-by: Sai Shree Pradhan <saishree.pradhan@databricks.com>

Author: saishreeeee

src/databricks/sql/thrift_backend.py

@@ -41,6 +41,7 @@
     convert_column_based_set_to_arrow_table,
 )
 from databricks.sql.types import SSLOptions
+from typing import Optional
 
 logger = logging.getLogger(__name__)
 
@@ -583,7 +584,7 @@ def open_session(self, session_configuration, catalog, schema):
             self._transport.close()
             raise
 
-    def get_session_id_hex(self) -> str:
+    def get_session_id_hex(self) -> Optional[str]:
         return self._session_id_hex
 
     def close_session(self, session_handle) -> None: