function normalizeAssertion(assertion) {
  if (!assertion) return ['off', {}];
  if (typeof assertion === 'string') return [assertion, {}];
  return assertion;
}
@param {LHCI.AssertCommand.AssertionFailureLevel | [LHCI.AssertCommand.AssertionFailureLevel, LHCI.AssertCommand.AssertionOptions] | undefined} assertion
@return {[LHCI.AssertCommand.AssertionFailureLevel, LHCI.AssertCommand.AssertionOptions]}