cli/dump: fails with column-less tables

**To Reproduce**

Given an empty database:

```
CREATE DATABASE foo;
CREATE TABLE bar();
```

```
$> cockroach dump --insecure foo
```

Results in:

```
CREATE TABLE bar (,
	FAMILY "primary" (rowid)
);
Error: pq: syntax error at or near "from"
Failed running "dump"
```

Quite evident in the example but can very frustating in a large database.

CC \ @knz 
