Fix accumulator methods

[belchior]

Accumulator methods don't remove calls with empty string and produces invalid sql syntax

use sql_quer_builder as sql;

let query = sql::Select::new()
  .select("")
  .select("login")
  .as_string();
let expected_query = "select login";

assert_eq!(query, expected_query);

failures:

Diff < left / right > :
<SELECT , login
>SELECT login