Merge pull request #890 from ngodfraind/bugfixes

NULL not merged to the em anymore.
Refactoring workspace creation.
Fixing a bug happening when importing a csv with no user role.
Merge pull request #891 from ngodfraind/master

Fixing a bug happening when importing a csv with no user role.
Merge branch 'master' of https://github.com/claroline/CoreBundle into bugfixes
Importers can add content to an existing workspace.