avoid NoSuchElementException if column is empty

```
java.util.NoSuchElementException: head of empty list
        at scala.collection.immutable.Nil$.head(List.scala:402)
        at scala.collection.immutable.Nil$.head(List.scala:399)
        at scalikejdbc.metadata.Table.toDescribeStyleString(Table.scala:68)
        at scalikejdbc.DB$$anonfun$describe$1.apply(DB.scala:796)
        at scalikejdbc.DB$$anonfun$describe$1.apply(DB.scala:796)
        at scala.Option.map(Option.scala:133)
        at scalikejdbc.DB.describe(DB.scala:796)
        at scalikejdbc.DB$.describe(DB.scala:318)
```