Check for empty row from mysql in PHP

I am trying to get a set of id from one table and get the details of the id from another table. Both tables are from 2 different database on two separate servers. I get an error when the data in the other table is empty.

In this example below, I stored all the user_id in an array called $user_data. Then, I would like to append to the existing array and add more info like first_name, last_name and email to the same user_id.

table: user

$user_data = array();

$sql_get_userid = "SELECT user_id FROM user
                   WHERE apply_date = ?";

$statement_get_id = $DB_3306->link->prepare($sql_get_userid);
$statement_get_id->bind_param("s", $date);
$statement_get_id->execute();

if($rs_get_id =  $statement_get_id->get_result())
{
    while($row = $rs_get_id->fetch_assoc())
    {
        $user_data[] = $row;
    }
}

There are times when the user_id might still exist but the data associated with the user_id in the other table is obsolete and deleted. Therefore, I get an error when I try to retrieve the non-existing data.

table: user_personal

$sql_get_user_data = "SELECT first_name, last_name, email FROM user_personal
WHERE user_id = ?"

foreach($user_id as $user => $child)
{ 
    $user_id = $child['user_id'];

    $statement_get_data = $DB_3308->link->prepare($sql_get_user_data);
    $statement_get_data->bind_param("s", $user_id);
    $statement_get_data->execute();

    if($rs_get_data = $statement_get_data->get_result())
    {
        while($row = $rs_get_data ->fetch_assoc())
        {
            if(isset($row['first_name']) && !empty($row['first_name']))
            {
               $first_name = $row['first_name'];
            }
            else
            {
                $first_name = 'NA';
            }
            $user_data[$user]['first_name'] = $first_name;
        }
    }
}

As you can see from the second part of the code, I try to check if the data is set and not empty and to add a message if it's empty. I still get an error "Notice: Undefined index". What did I do wrong here?

EDIT: Actually the $user_data doesn't even append the data if the other table doesn't have it.

//A rough idea of the data stored in $user_data
$user_data = Array( [0] => Array ( [user_id] => 111), [1] => Array ( [user_id] => 222) [2] => Array ( [user_id] => 333))

//If user_id = 333 has no data associated with it in user_personal table the array would look like this (showing only first_name value)
 $user_data = Array( [0] => Array ( [user_id] => 111 [first_name] => Kenny ), [1] => Array ( [user_id] => 222 [first_name] => Kenny) [2] => Array ( [user_id] => 333))

Actually the array from the codes here are called to an display file where the data are displayed. So, I moved the if(isset(var) && !empty(var)) to that file and so far it works fine.