message: Do not prefill a value for a checkbox
  If the BooleanField in the given resource is not set, the tristate
  logic would set it to false by default. This commit changes that to
  differentiate between the value that is shown (false), and the
  value that's actually present on the web.
diff: @@ -1202,20 +1202,22 @@ function buildCheckboxForm (dom, kb, lab, del, ins, form, store, tristate) { //
  }
  function refresh () {
  var state = holdsAll(ins)
  + var displayState = state
  if (del.length) {
  var negation = holdsAll(del)
  if (state && negation) {
  - box.appendChild(error.errorMessageBlock(dom,
  + box.appendChild(UI.widgets.errorMessageBlock(dom,
  'Inconsistent data in store!\n' + ins + ' and\n' + del))
  return box
  }
  if (!state && !negation) {
  + state = null
  let defa = kb.any(form, UI.ns.ui('default'))
  - state = defa ? defa.value === '1' : tristate ? null : false
  + displayState = defa ? defa.value === '1' : tristate ? null : false
  }
  }
  input.state = state
  - input.textContent = {true: checkMarkCharacter, false: cancelCharacter, null: dashCharacter}[input.state]
  + input.textContent = {true: checkMarkCharacter, false: cancelCharacter, null: dashCharacter}[displayState]
  }
  refresh()
  