title: NpgsqlDataReader - distinguish null value from "NULL" literal
body: <p>I have a problem with distinguishing NULL database type from "NULL" text when reading a table column from database</p>
  
  <p>Sample code:</p>
  
  <pre><code>using (var connection = new NpgsqlConnection("connectionString"))
  {
      connection.Open();
      var query = @"SELECT array_agg(a) col FROM unnest('{NULL,""NULL""}'::text[]) a";
      using(var cmd = new NpgsqlCommand(query, connection)
      {
          using(var reader = cmd.ExecuteReader())
          {
              while (reader.Read())
              {
                  var strings = rowa["col"] as string[];
                  Console.WriteLine(strings[0]);
                  Console.WriteLine(strings[1]);
              }
          }
      }
  }
  </code></pre>
  
  <p>both <code>strings[0]</code> and <code>strings[1]</code> are <code>"NULL"</code> but the first one should be a null value</p>