Check if decoded JSON returned null
Check if decoded JSON returned null

The value might return null resulting in a PHP error.

Fixes https://github.com/owncloud/core/issues/19542

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