folder_branch_ops: explicit nil err when rekeying a team TLF
folder_branch_ops: explicit nil err when rekeying a team TLF

Suggested by jzila.

Issue: #1974

keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client