[sql/sem/tree]: Pretty-printed tuples are always formatted with comma

We're using the `tree` package to format auto-generated SQL for tests. It's very useful for this, thanks for the work that has gone into this package.

One thing that we came across is that tuples are formatted with a trailing comma when there's a single item in it:

https://github.com/cockroachdb/cockroachdb-parser/blob/302c9ad52e1a81d9cf51398875c45333a1c6d635/pkg/sql/sem/tree/pretty.go#L1124-L1126

While this is equivalent, the output is a bit unexpected

Input

```sql
SELECT * FROM some_table WHERE (some_column IN ('abc'))
```

Output

```sql
SELECT * FROM some_table WHERE some_column IN ('abc',);
```