in shelfs inventory page, fix empty value in column Debut inventaire en cours…

in shelfs inventory page, fix empty value in column Debut inventaire en cours (bug introduced after v12 port)

shelfs/models.py

@@ -566,7 +566,7 @@ class Shelfs(models.Model):
             for r in res:
                 if precision != 'simple':
                     if r['ongoing_inv_start_datetime'] == "1-01-01 00:00:00":
-                        r['ongoing_inv_start_datetime'] = ""
+                        r['ongoing_inv_start_datetime'] = default_inventory_start_datetime
                     if r['date_last_product_added'] == "1-01-01":
                         r['date_last_product_added'] = ""
                     coop_logger.info(str(r))