The developers MUST care for the quality, not outsource it to some external team. The desire of large corps to spend huge amount of $$$ to create artificial roles to water down ownership can never cease to astonish me.
If your devs can't think of handling nulls or corner cases, you got a skill problem, and you should ASAP upskill the devs, not outsource thinking to another tea.
Making quality somebody else's problem is the exact opposite of ownership! You want agile? Damn leave the devs alone, stop building walls around them and the product they are building!