User should be notified in case he has not specified any targets BUT has specified task-level options

If you specify a task with only task-level options but no targets, e.g.:

```
mytask: {
  options: {
    stuff: 'things';
  }
}
```

Grunt should tell you that no targets were found, which is not the case.

HOWEVER, when you do the following, it actually errors out saying 'No "mytask" toggles targets found.':

```
mytask: {
}
```

The behaviour for the two snippets shown above should be consistent (i.e. informing the user that no targets are available.)