What is the default value for $operator in condition()?
QueryConditionInterface is just that, an interface. The actual class used is DatabaseCondition.

In DatabaseCondition:condition(), it does set the default operator:

if (!isset($operator)) {
  if (is_array($value)) {
    $operator = 'IN';
  }
  elseif (!isset($value)) {
    $operator = 'IS NULL';
  }
  else {
    $operator = '=';
  }
}