[FIX] Prevent NPE in SarosProjectMapper

This Bug was introduced in commit
609885ac8a6518b3c9c3771560989cf2d5ff2dcb. Happened when Bob sends
IRessourceActivities while Carl is still in SessionNegotiation.

Change-Id: I834165fb26f3cbc70c1a651d6012ef2356bdff4a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/882
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>