/**
  * Process an property access expression node for nullability
  * 
  * @param PropertyAccessExpression $expression            The property access 
  *                                                        expression node to process.
  * @param AbstractExpression       $parentExpression      The parent expression of 
  *                                                        expression node to process.
  * @param boolean                  $checkNullForMostChild Wheter to check null for 
  *                                                        most child node or not.
  * 
  * @return LogicalExpression, RelationalExpression or NULL
  */
 private function _processPropertyAccessNode(PropertyAccessExpression $expression, $parentExpression, $checkNullForMostChild)
 {
     $navigationsUsed = $expression->getNavigationPropertiesInThePath();
     if (!empty($navigationsUsed)) {
         $this->_navigationPropertiesUsedInTheExpression[] = $navigationsUsed;
     }
     $nullableExpTree = $expression->createNullableExpressionTree($checkNullForMostChild);
     if ($parentExpression == null) {
         return new LogicalExpression($nullableExpTree, $expression, ExpressionType::AND_LOGICAL);
     }
     return $nullableExpTree;
 }