Undefined offset:0 at HandleExceptions->handleError - ajax

Question

I have a page which is used to search the users. If I searched an existing user, the page will redirect to my wanted page. If the user is not existing, a message must be shown. But for me the message is not showing. I know may be this is a simple error, but it's not working for me. I am using Laravel 5.8.
Error is:
Undefined offset: 0
in ClientsController.php (line 344)
at HandleExceptions->handleError.
Here is my controller:
public function getMerchantDetails() {
$user = $_GET['user'];
$mid = $_GET['mid'];
$activity = "User $user has searched this merchant";
//This is for Activity Log Capturing. Added user variable
DB::insert("INSERT INTO activitylog (mid,activity) values ('$mid','$activity')");
//Activity ends here
$mobile = $_GET['mobile'];
$email = $_GET['email'];
$m = '';
if ($mobile != '') {
$m = " AND primary_number = '" . $mobile . "' ";
}
$e = '';
if ($email != '') {
$e = " AND email = '" . $email . "' ";
}
$i = '';
if ($mid != '') {
$i = " AND mid = '" . $mid . "' ";
}
if ($m == '' && $e == '' && $i == '') {
// print_r($m);
// die();
return response()->json(array('data' => ''), 200);
} else {
$res = DB::select(DB::raw("SELECT * FROM clients WHERE 1=1 " . $m . $e . $i . " LIMIT 1"));
if (!$res) {
//$res_config = DB::select( DB::raw("SELECT * FROM configuration WHERE code = 'PERL_SEARCH_MERCHANT'") );
$perl_path = DB::select(DB::raw("SELECT * FROM configuration WHERE code = 'PERL_PATH'"));
$perl_output = exec($perl_path[0]->description . ' -merchant_id ' . $mid . ' -mobile ' . $mobile . ' -email ' . $email);
$r = json_decode($perl_output, true);
if ($r['status'] == 'success') {
if ($r['id'] != '') {
$res = DB::select(DB::raw("SELECT * FROM clients WHERE id = " . $r['id']));
} else {
$res[0]['id'] = '';
}
}
}
return response()->json(array('data' => $res[0]), 200); (line 344)
}
}
My Ajax code:
$.ajax({
"url": '{!! route('clients.fetchMerchantDetails') !!}',
async: true,
type: "GET",
data: {
mobile: mobile,
email: email,
mid: mid,
user:userid
},
success: function (response) {
var data = response.data
if(data.id!=''){
clientEditUrl = clientEditUrl.replace(':id', data.id);
window.location = clientEditUrl;
}else{
$('.searchOutputDiv').html('<h3>No merchant found!</h3>');
}
}
});