mortido,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
$(".btn").on("click", function() {
var reg = /^\s*$/,
test = [].some.call(document.querySelectorAll(".add_site_inp"), function(el) {
return reg.test(el.value)
});
if (test) alert("Заполните все поля!")
})
});
</script>
</head>
<body>
<input type="text" class="add_site_inp" value="1">
<input type="text" class="add_site_inp" value="2">
<input type="text" class="add_site_inp" value="3">
<input type="text" class="add_site_inp" value="4">
<input type="text" class="add_site_inp" value="">
<input type="text" class="add_site_inp" value="6">
<input type="text" class="add_site_inp" value="7">
<input name="" type="button" value="test" class="btn">
</body>
</html>