How to determine if a found object is empty?

The code below, which I tested with alter,

The left popup is "empty".　The right popup is  [objet charater].

This shows that there are  difference.    Since it is different, the choice can be judged.

 But when I use if(!qz===''),  It always prompts (!qz===‘’) with a result of false.

What do I have to write   "if (   )"  to get it right?

Another problem:
If an @ is found, and there is only one, also execute the subsequent code.
If the loop does not find an @, it prompts alert(“No one @”).

How  to write this?

Thank you very much.

    for (var h = 0, k = 0; h < sa.length; h++) {
        if ("TextFrame" == sa[h].constructor.name) {
            if (sa[h].overflows) {
                sa[h].fit(FitOptions.FRAME_TO_CONTENT)
            }
            //alert(sa[h]);
            var qz = getGrepSearch("@", sa[h]);
            alert(qz);
            alert((!qz === ''));
            //for (var k = 0; k < sa.length; k++) {
            if (qz === !"") {