Optional GROUP BY statement

Working on my custom sql validation, I needed somehow to check, does user provide group_by/limit+offset/sort_by clauses.
I got smth like this and found `.group_by` field as non-optional. Is it right?

```rust
let wrong_query = "SELECT * FROM example_table GROUP BY example_field";
//Parser returns success(!) below
if let Statement::Query(q) = Parser::parse_sql(&GenericDialect {}, &query)?.first().unwrap() {
if q.limit_clause.is_none() {...}
if q.order_by.is_none() {...}
let select_query = query.body.as_select().unwrap();
//And I expected smth like this:
if select_query.group_by.is_none() {...}
}

```

Maybe I'm doing smth wrong and there is a proper and faster way for that?
Also I noticed a `parse_optional_group_by` fn but I didn't get how to use it., there is no info nor examples in docs about it. Looks like it's inner fn?

Start by reading the Select.group_by field returned through Parser::parse_sql and the parse_optional_group_by function mentioned in the issue. Determine how the parser represents an absent GROUP BY clause and whether the public API or its documentation needs to expose that distinction; done means the intended behavior is documented or covered by an appropriate parser test.

索引モデルが issue の本文から書いたものです。