[REFACTOR] RosterSessionContentProvider - removed redundant null checks

Cleaned up the code so it should much more readable now.

Change-Id: I97f5184044c998b4a80668c5946fc8d78ea63e63
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1178
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>