[SQL] Minor HashAggregateExec string output fixes

## What changes were proposed in this pull request?

This PR fixes some minor `.toString` format issues for `HashAggregateExec`.

Before:

```
*HashAggregate(key=[a#234L,b#235L], functions=[count(1),max(c#236L)], output=[a#234L,b#235L,count(c)#247L,max(c)#248L])
```

After:

```
*HashAggregate(keys=[a#234L, b#235L], functions=[count(1), max(c#236L)], output=[a#234L, b#235L, count(c)#247L, max(c)#248L])
```

## How was this patch tested?

Manually tested.

Author: Cheng Lian <lian@databricks.com>

Closes #13710 from liancheng/minor-agg-string-fix.