remove success toast for parcours startdate + take empty string values into account for action dates

src/Bundle/ChillPersonBundle/Resources/public/vuejs/AccompanyingCourse/components/StartDate.vue

@@ -28,14 +28,13 @@ export default {
             const date = e.target.previousSibling.value;
 
             this.$store.dispatch('updateStartDate', date)
-                .then(this.$toast.open({type: 'success', message: this.$t('startdate.update')}))
                 .catch(({name, violations}) => {
                     if (name === 'ValidationException' || name === 'AccessException') {
                         violations.forEach((violation) => this.$toast.open({message: violation}));
                     } else {
                         this.$toast.open({message: 'An error occurred'})
                     }
-            });
+            })
         },
     },
     computed: {