[FIX] Hooks now correctly check if counterside implemented hook

All existing hooks now properly handle null values.

Change-Id: I3125f757c98f005d1bd78255c8d8e4b21bbaa60a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1718
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>