Problem handling null value with jdbc_streaming filter

[Kowsalya_Mouttou (Kowsalya Mouttouramane)]

Hi, here is my filter plugin in logsatsh configuration :

filter {
	jdbc_streaming {
    		jdbc_driver_library => "/usr/share/logstash/logstash-core/lib/jars/postgresql-42.2.6.jar"
    		jdbc_driver_class => "org.postgresql.Driver"
    		jdbc_connection_string => "jdbc:postgresql://*******/***"
    		jdbc_user => "*****"
    		jdbc_password => "****"
    		statement => "SELECT dir.libelledir, dir.datedebut, dir.codedeleg, deleg.libelledeleg FROM gaia2stat.t_direction AS dir, gaia2stat.t_delegation AS deleg
		 WHERE dir.codedir = :codedir_param AND dir.codedeleg = deleg.codedeleg "
		parameters => { "codedir_param" => "codedir"}
		target => "dir_deleg"
		default_hash => { 
			"libelledir" => ""	
			"datedebut" => ""
			"codedeleg" => ""
			"libelledeleg" => ""
			}
		add_field => {
			"libelledir" => "%{[dir_deleg][0][libelledir]}"	
			"datedebut" => "%{[dir_deleg][0][datedebut]}"
			"codedeleg" => "%{[dir_deleg][0][codedeleg]}"
			"libelledeleg" => "%{[dir_deleg][0][libelledeleg]}"
			}
		#remove_field => ["dir_deleg"]
  	}

Even after setting the default_hash option, i have problem handling null values in the "datedebut" field.
I've %{[dir_deleg][0][datedebut]} in some of my documents instead of "" for the "datedebut" field.
Can anyone please help me solving this issue ?