fix: handle NULL parrain column in GetClientParrain

sql.NullString permet de scanner une valeur NULL depuis la colonne
parrain sans erreur de conversion.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/gestion/db/db_parrain.go

@@ -1,6 +1,7 @@
 package db
 
 import (
+	"database/sql"
 	"fmt"
 	"gestion/models"
 )
@@ -19,12 +20,15 @@ func (d *Database) SetClientParrain(clientUsername, parrainUsername string) erro
 }
 
 func (d *Database) GetClientParrain(clientUsername string) (string, error) {
-	var parrain string
+	var parrain sql.NullString
 	err := d.GDB.Table("clients").
-		Select("COALESCE(parrain, '')").
+		Select("parrain").
 		Where("username = ?", clientUsername).
 		Scan(&parrain).Error
-	return parrain, err
+	if err != nil {
+		return "", err
+	}
+	return parrain.String, nil
 }
 
 func (d *Database) GetClientsByParrain(parrainUsername string) ([]models.Client, error) {