IF condition check inside USING method and SqlConnection

I am trying to run data validation, execute some code and pass data from one sql query to another.

My Current code looks like the below:

        public string SelectUniqueKeyNumber()
    {
        string newList = string.Join(Environment.NewLine, listOfSkus).ToString();
        string key_id;
        string sqlConnectionString = @"someConnectionString";
        using (SqlConnection connection = new SqlConnection(sqlConnectionString))
        {
            connection.Open();
            SqlCommand command = new SqlCommand("select top 1 KEY_NUMBER from MyTable where QTY_ON_HAND > 0 " + newList + " order by NEWID()", connection);
            SqlDataReader readerKey = command.ExecuteReader();
            readerKey.Read();
            key_id = String.Format(readerKey[0].ToString());
        }
        SelectSkuNumber(key_id);
        return key_id;
    }

What I am trying to do is to check if my readerKey.Read() is not returning null value. If it does then stop the process, otherwise continue. I've tried it in the way as shown below:

    public string SelectUniqueKeyNumber()
{
    string newList = string.Join(Environment.NewLine, listOfSkus).ToString();
    string key_id;
    string sqlConnectionString = @"someConnectionString";
    using (SqlConnection connection = new SqlConnection(sqlConnectionString))
    {
        connection.Open();
        SqlCommand command = new SqlCommand("select top 1 KEY_NUMBER from MyTable where QTY_ON_HAND > 0 " + newList + " order by NEWID()", connection);
        SqlDataReader readerKey = command.ExecuteReader();
        readerKey.Read();
        if(readerkey.Read().ToString() == null)
        {
            //--- stop processing
        }
        else
        {
            key_id = String.Format(readerKey[0].ToString());
        }
    }
    SelectSkuNumber(key_id); //---> Then ...(key_id) is not declared value
    return key_id;
}

By doing so, I cannot access and pass data of SelectSkuNumber(key_id) due to: Use of unassigned local variable 'key_id'

Any ideas?