fix: type error with progress updater

TypeError: unsupported operand type(s) for +=: 'int' and 'NoneType'

Resolve BE-2133

sdk/src/beta9/multipart.py

@@ -148,7 +148,7 @@ def target():
             except Exception:
                 continue
 
-            finished += processed
+            finished += processed or 0
             if callback is not None:
                 callback(total=file_size, advance=processed)