Fix for intermittent failing previews tests

Fixes:
```
Preview processor Preview generation verify reprocessing a preview processes the revision preview:
     Uncaught TypeError: Cannot read property 'alias' of undefined
```

This bug was caused because the previous test submitted `c:camtest:someContent` and `c:camtest:someRevision`
which passed validation but made the PP trip up.

When a content and/or revision cannot be found, the PREVIEWS_FINISHED event will not be emitted.