executemany crashes with empty input

[dvarrazzo]

Regression in 3.0.6

    def test_executemany_no_data(conn, execmany):
        cur = conn.cursor()
>       cur.executemany("insert into execmany(num, data) values (%s, %s)", [])

tests/test_cursor.py:272: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
psycopg/psycopg/cursor.py:569: in executemany
    self._conn.wait(self._executemany_gen(query, params_seq))
psycopg/psycopg/connection.py:773: in wait
    return waiting.wait(gen, self.pgconn.socket, timeout=timeout)
psycopg/psycopg/waiting.py:243: in wait_epoll
    s = gen.send(ready)
psycopg/psycopg/cursor.py:225: in _executemany_gen
    self._set_result(0)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <psycopg.Cursor [no result] [INTRANS] (port=54314 user=piro database=psycopg3_test) at 0x7feca6b5cd50>, i = 0, format = None

    def _set_result(self, i: int, format: Optional[Format] = None) -> None:
        """
        Select one of the results in the cursor as the active one.
        """
        self._iresult = i
>       res = self.pgresult = self._results[i]
E       IndexError: list index out of range

psycopg/psycopg/cursor.py:410: IndexError