Per discussion in another thread, the in-game help says any member can take over a guild after 11 days leader absence. It has been over 11 days for my guild Walkers in Twin Peak with a “blank” leader. I do not see a way to claim leadership. Other guilds in the list are also still blank after more than 11 days, so they may have the same problem.
If it happens by personal request to devs, I am willing to take leadership of Walkers for now. 
ID w8y6ye