fix a bug when there's no element on page

e.g.

    input[checkbox]:checked{
        abc:abc;
    }

when no checkbox on page, it will output:

   /*some css 1*/

    {
        abc:abc;
    }

    /*some css 2*/

thus makes 'some css 2' invaild.


Another case:

    input[checkbox]:checked,input[checkbox]:disabled:checked{
        abc:abc;
    }

if no checkbox is disabled, it will output:

    /*some css 1*/

    CPS0,CPS1,{
        abc:abc;
    }

    /*some css 2*/

makes the selector is invalid.