Empty update fails with a helpful error:

  $ setconfig ui.disallowemptyupdate=True
  $ newrepo
  $ hg debugdrawdag <<'EOS'
  > B
  > |
  > A
  > EOS
  $ hg up -q 0
  $ hg up
  abort: You must specify a destination to update to, for example "hg update master".
  (If you're trying to move a bookmark forward, try "hg rebase -d <destination>".)
  [255]

up -r works as intended:
  $ hg up -q -r 1
  $ hg log -r . -T '{rev}\n'
  1
  $ hg up -q 1
  $ hg log -r . -T '{rev}\n'
  1