Fix UnboundLocalError in ZipfileBackendRepository (#7129)

If a file/key is missing when importing an archive, the `open` method in
`ZipfileBackendRepository` fails with an `UnboundLocalError` because the
`handle` variable is referenced before assignment.

This was initially discovered when working on the following issue on
discourse: https://aiida.discourse.group/t/error-archive-import/696

(cherry picked from commit 166d06c)

Author: t-reents

src/aiida/storage/sqlite_zip/backend.py

@@ -441,13 +441,15 @@ def list_objects(self) -> Iterable[str]:
 
     @contextmanager
     def open(self, key: str) -> Iterator[BinaryIO]:
+        handle = None
         try:
             handle = self._zipfile.open(f'{self._folder}/{key}')
             yield cast(BinaryIO, handle)
         except KeyError:
             raise FileNotFoundError(f'object with key `{key}` does not exist.')
         finally:
-            handle.close()
+            if handle is not None:
+                handle.close()
 
 
 class FolderBackendRepository(_RoBackendRepository):