Search for persons with no user assigned

Closes #298.

When a search was made and the results contai some user that has no user assigned the search result was not shown.

We now verify if the user is null before trying to read his username.

As all the ids of a user can be null, the way in which they are present (separated by comma) would have been more complex to implement (lots of IFs). I've refactored the code so that we can use the Guava Joiner to join all the user ids.