[INTERNAL] changed instanceof argument

It is possible that the passed ISarosSession is actually a
NullSarosSession which should not be accessed (see warnings in the log
file).

Change-Id: Id9377d71bc2b9d3bd5b5e0a4232360f39c591748
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1311
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>