The Null were among the alien forces stopped by UNIT in the early days of the organisation. (AUDIO: Old Soldiers) Some historians believed that the Clades were created in response to attacks from the Null. (PROSE: Peacemaker)
Behind the scenes
As far as this wiki is currently aware, all mentions of the Null in Doctor Who fiction occur in stories written by James Swallow. The Null featured in Swallow's 2009 Star Trek novel Synthesis.