Empty arrays

Postgres's array constructor syntax won't let you construct an empty array without specifying its element type (like `ARRAY[]`):

```
ERROR:  cannot determine type of empty array
HINT:  Explicitly cast to the desired type, for example ARRAY[]::integer[].
```

(`ARRAY[1,2,3]` with no cast is fine, I guess because Postgres can derive the type.)

`'{}'` (the Postgres array literal syntax) works fine, however, so this patch updates S-SQL to use that when the vector is empty.

An alternative approach would be to have S-SQL try to look at the Lisp element-type of the vector and figure out the right cast, but that seems like needless complexity.