<TRANSITION from="In Progress" to="Ready For Test">
          <REASONS>
            <DEFAULTREASON value="Ready for testing" />
          </REASONS>
          <FIELDS>
            <FIELD refname="System.AssignedTo">
                <ALLOWEDVALUES expanditems="true">
                    <LISTITEM value="[project]\QA" />
                </ALLOWEDVALUES>
              <COPY from="value" value="Team member" />
            </FIELD>
          </FIELDS>
          <ACTIONS>
            <ACTION value="Microsoft.VSTS.Actions.Checkin" />
            <ACTION value="Microsoft.TeamSystem.Actions.Checkin" />
          </ACTIONS>
        </TRANSITION>

category.ParentCategoryId = @parentCategoryId

 or

category.ParentCategoryId = null and this sql didn't return result, because expect category.ParentCategoryId is null.

Then I test another way to delive null value 

 if (parentCategoryId == null)
            {
                var query = from
                                    category in this.productsHelpDataContext.Categories
                            orderby
                                    category.Order ascending
                            where
                                category.ParentCategoryId == null

                            select category;

                return query.Select(category => category);
            }
            else
            {
                var query = from
                                    category in this.productsHelpDataContext.Categories
                            orderby
                                    category.Order ascending
                            where
                                category.ParentCategoryId == parentCategoryId
                            select category;

                return query.Select(category => category);
            }

In other word LINQ to SQL strict follow sql script convention.

DECLARE @TEST VARCHAR(30)
SET @TEST = '1,2,3,47, 89, 67'
DECLARE @Current INT


DECLARE @LISTINT TABLE (Number INT)

WHILE (LEN(@TEST) > 0)
BEGIN
    IF (Charindex(',', @TEST) > 0)
    BEGIN
        SET @Current = LEFT(@TEST, Charindex(',', @TEST) - 1)
        
        SET @TEST = RIGHT(@TEST, LEN(@TEST) - Charindex(',', @TEST))
        
    END
    ELSE
    BEGIN
        SET @Current = CAST(@TEST AS INT)
        SET @TEST = NULL
    END

    INSERT INTO @LISTINT VALUES(@Current)
    SET @Current = NULL
END

SELECT Number
FROM @LISTINT