key_bundle: add IsNil method on TLF{Writer,Reader}KeyBundleID
key_bundle: add IsNil method on TLF{Writer,Reader}KeyBundleID

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/kbfs,keybase/client,keybase/kbfs