fix a bug when there's no element on page

[TooBug]

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.