Fix getsize benchmark

[dcherian]

Zarr doesn't support a preceding slash either, so I think icechunk was mistakenly supporting it.

import zarr
store = zarr.storage.MemoryStore()

group = zarr.group(store)

array = group.create_array("array", shape=(1,2,3), dtype="float32")

print(await store.getsize("array/zarr.json")) # works
await store.getsize("/array/zarr.json") #error