Druid adapter: "GROUP BY ()" on empty relation should return 1 row

A "GROUP BY ()" query on an empty relation should return 1 row, but currently returns 0 rows.

Test case in DruidAdapterIT:

  @Test public void testSelectCountEmpty() {
    sql("select count(*) as c from \"foodmart\" where \"product_id\" < 0")
        .returnsUnordered("C=0");
  }

The query should return one row, but returns 0 rows.