in get_all shelfs in precision simple case, do not try to access missing fields

shelfs/models.py

@@ -564,6 +564,7 @@ class Shelfs(models.Model):
             else:
                 res = api.execute('product.shelfs', 'get', {})
             for r in res:
+                if precision != 'simple':
                     if r['ongoing_inv_start_datetime'] == "1-01-01 00:00:00":
                         r['ongoing_inv_start_datetime'] = ""
                     if r['date_last_product_added'] == "1-01-01":