Allow stocktake date to be empty on part lot form

Fixes issue #1250

src/Form/Part/PartLotType.php

@@ -117,7 +117,6 @@ class PartLotType extends AbstractType
             'widget' => 'single_text',
             'disabled' => !$this->security->isGranted('@parts_stock.stocktake'),
             'required' => false,
-            'empty_data' => null,
         ]);
     }