handle empty string prefix in zipstore delete dir (#2492)

Author: d-v-b

src/zarr/storage/zip.py

@@ -215,7 +215,7 @@ async def set_if_not_exists(self, key: str, value: Buffer) -> None:
     async def delete_dir(self, prefix: str) -> None:
         # only raise NotImplementedError if any keys are found
         self._check_writable()
-        if not prefix.endswith("/"):
+        if prefix != "" and not prefix.endswith("/"):
             prefix += "/"
         async for _ in self.list_prefix(prefix):
             raise NotImplementedError