not really. Type: Null cannot be a tank because of a number of reasons. 1. if you look at it's stats, they are all the same except for the SPEED stat, which is pathetic compared to Silvally. 2. both TN and Silvally are actually attackers. it has no actual defensive moves in it's move pool. the only defensive move it has would be defense curl. 3. Silvally is AMAZING!!! i use one and it's like a raptor, it gets in then gets out fast. And you have the Memories that Gladion gave you, so you can make Silvally any type to balance out your team.
My advice: evolve TN and if you really want to try out a tank TN, try to find one on the GTS or try to make an egg. As far as i know, Silvally can breed, i think. im trying to do that now actually. im trying to find a Hardy natured Ditto, since Hardy is one of the universal natures of breeding. So evolve TN. It can't be used as a tank, it doen't have the stats for it.