Use correct === instead of is_null()

When checking if a WBO is already in the database, use the correct ```
fetchRow() === false``` instead of ```is_null()```. The function
```fetchRow()``` returns the boolean ```false``` when no (more) row(s)
are in the result set.
Merge pull request #99 from owncloud/correct_wbo_insert_update_check

Use correct === instead of is_null()