should groups be allowed to self-annihilate

[d-v-b]

See this example:

>>> from zarr import create_group
>>> create_group({})
<Group memory://126474867787584>
>>> g = create_group({})
>>> g.store._store_dict
{'zarr.json': <zarr.core.buffer.cpu.Buffer object at 0x73073576ef50>}
>>> del g['']        
>>> g.store._store_dict 
{}

There are two issues here -- first, should this be allowed at all? I think not, which means we need to make some changes to how groups resolve node names. Second, how should we handle situations where an in-memory group or array is backed by an invalid or stale stored representation? Would anyone find value in a function that checks whether an array or group is consistent with its stored representation?