Check the expiration date for null
Check the expiration date for null

* null is always less than any value -> expirationDate gets null
  which is "no date set"
* ref https://github.com/owncloud/core/issues/20590#issuecomment-158393075

jbicha/server,michaelletzgus/nextcloud-server,Ardinis/server,andreas-p/nextcloud-server,owncloud/core,cernbox/core,whitekiba/server,jbicha/server,andreas-p/nextcloud-server,endsguy/server,whitekiba/server,xx621998xx/server,endsguy/server,bluelml/core,pixelipo/server,michaelletzgus/nextcloud-server,pixelipo/server,endsguy/server,nextcloud/server,michaelletzgus/nextcloud-server,owncloud/core,pollopolea/core,owncloud/core,sharidas/core,whitekiba/server,Ardinis/server,cernbox/core,endsguy/server,pollopolea/core,whitekiba/server,pixelipo/server,nextcloud/server,Ardinis/server,IljaN/core,nextcloud/server,bluelml/core,whitekiba/server,pmattern/server,endsguy/server,owncloud/core,jbicha/server,xx621998xx/server,pixelipo/server,cernbox/core,bluelml/core,sharidas/core,IljaN/core,pollopolea/core,IljaN/core,sharidas/core,IljaN/core,jbicha/server,IljaN/core,xx621998xx/server,andreas-p/nextcloud-server,phil-davis/core,Ardinis/server,Ardinis/server,jbicha/server,michaelletzgus/nextcloud-server,pmattern/server,pollopolea/core,pollopolea/core,pmattern/server,owncloud/core,sharidas/core,xx621998xx/server,sharidas/core,pixelipo/server,cernbox/core,cernbox/core,andreas-p/nextcloud-server,nextcloud/server,pmattern/server,pmattern/server,xx621998xx/server,bluelml/core,andreas-p/nextcloud-server,bluelml/core