Issue #401: min(balance) prints empty column (blais/beancount)

[adamdblu]

New issue 401: min(balance) prints empty column
https://bitbucket.org/blais/beancount/issues/401/min-balance-prints-empty-column

Adam Wolenc:

```
beancount> SELECT date, balance WHERE account = "Assets:Bank"
date balance
---------- --------
2019-01-01 1.00 USD
2019-01-01 2.00 USD
```

```
beancount> SELECT date, max(balance) WHERE account = "Assets:Bank" GROUP BY date
date max_bala
---------- --------
2019-01-01 2.00 USD
```

```
beancount> SELECT date, min(balance) WHERE account = "Assets:Bank" GROUP BY date
date
---------- -
2019-01-01
```

Expected: 1.00 USD