hash-sets,hashtables: make it so the array backing the hash is non-empty

hash-sets,hashtables: make it so the array backing the hash is non-empty

Commit 70c7f9e02910746918cc16bf588ef543fda80790 made it so the code
assumes the array is not empty. But it can be empty if the hashtable is
created using "hashtable new" and then it can crash because it reads
uninitialized memory. Setting the initial of the array slot to
a valid hash-array should fix that.