Solved the chart error when no data available.

    When preparing the array for the chart, the function tries to parse
    the json for each blacklisted website storage object. But if no such
    object is available, a parse error is thrown and the chart is not
    draw.

    In the initialization function we are checking if every blacklisted
    website has an storage object. If not, this object is created. Till
    now this check was made when the user accessed one of the
    blacklisted sites.

    When using for the first time the extension, there is no data. Due
    to the initialization proccess described above, the user will be
    presented with a table listing all the blacklisted websites with
    value "0" for the wasted amount.
    Also, the charts will draw 0 value bars.