Inner Join only if satisfies If condition
I did something like this

CREATE PROCEDURE [dbo].[spGetItemCategory] (@ShopID INT)
AS
BEGIN
    SET NOCOUNT ON;

    SELECT ItemCategory.ItemCategoryID
        ,CategoryName
    FROM ItemCategory
    INNER JOIN ShopitemCategory ON (
            ShopitemCategory.shopid = @ShopID
            AND ShopitemCategory.itemcategoryid = ItemCategory.ItemCategoryID
            )
        OR (@ShopID = 0)
END