Support nullary products

[alexfmpe]

Needed this alongside haskell-beam/beam#563

[alexfmpe]

This probably needs to be reworked given upstream concerns

[cgibbard]

We should see about the potential for completely empty tables. I believe tables without columns are allowed in postgresql, and obviously, they're a perfectly cromulent notion, but they may not be permitted in some versions of SQL.

[cgibbard]

psql (9.6.15)
Type "help" for help.

postgres=# CREATE TABLE zero ();
CREATE TABLE
postgres=# \d zero
    Table "public.zero"
 Column | Type | Modifiers 
--------+------+-----------

postgres=# insert into zero values ();
ERROR:  syntax error at or near ")"
LINE 1: insert into zero values ();
                                 ^
postgres=# insert into zero default values;
INSERT 0 1
postgres=# SELECT * FROM zero;
--
(1 row)

postgres=# insert into zero default values;
INSERT 0 1
postgres=# SELECT * FROM zero;
--
(2 rows)

[cgibbard]

I think this may require a backend-specific check to see if the resulting tables are sensible.