jQuery CSS Disable & Enabled Button dengan Text

<style>
#msg{
    font-family:arial;
    font-size:13px;
    color:#333;
    border: 1px solid #999;
    padding:3px;
    margin:0;
}
#kirim,#reset{
    border:1px solid #999;
    background:#088000;
    color:#fff;
    font-family:arial;
    font-size:13px;
    display:inline-block;
    margin:0;
    padding:6px 12px;
}
#kirim[disabled]{
    color:#999;
    background:#4B6146;
}
</style>
<script src="https://code.jquery.com/jquery-1.8.3.min.js"></script>
<script>
$(function(){
$('#msg').keyup(function(){
    if($(this).val().length>0){
        $('#kirim').attr('disabled',false);
    }else{
        $('#kirim').attr('disabled',true);
    }
});
});
</script>
<form>
<textarea id="msg" cols=20 rows=3></textarea><br />
<input id="reset" type="reset" class="button" value="Hapus" />
<input id="kirim" type="submit" class="button" value="Kirim" disabled />
</form>