No need to show common values if everything is null

Noticed this:

```
% sqlite-utils analyze-tables content.db repos -c delete_branch_on_merge --common-limit 20 --no-least
repos.delete_branch_on_merge: (1/1)

  Total rows: 158
  Null rows: 158
  Blank rows: 0

  Distinct values: 0

  Most common:
    158: None
```

The `158: None` there is duplicate information considering we already know there are 158/158 null rows.