Fix failing validation in Registration
Fix failing validation in Registration

In the conference is not set, validation that the call for paper is
accepted breaks. This made the conference association test failing.

differentreality/osem,SeaGL/osem,hennevogel/osem,openSUSE/osem,openSUSE/osem,AndrewKvalheim/osem,differentreality/osem,hennevogel/osem,SeaGL/osem,openSUSE/osem,differentreality/osem,AndrewKvalheim/osem,AndrewKvalheim/osem,hennevogel/osem,SeaGL/osem,differentreality/osem,hennevogel/osem