[24613] Fix unsetting date in datepickers

frontend/app/components/input/transformers/transform-date.directive.ts

@@ -37,13 +37,13 @@ function transformDate(TimezoneService:any) {
       ngModelController:any) {
       ngModelController.$parsers.push(function(data:any) {
         if (!moment(data, 'YYYY-MM-DD', true).isValid()) {
-          return undefined;
+          return null;
         }
         return data;
       });
       ngModelController.$formatters.push(function(data:any) {
         if (!moment(data, 'YYYY-MM-DD', true).isValid()) {
-          return undefined;
+          return null;
         }
         var d = TimezoneService.parseDate(data);
         return TimezoneService.formattedISODate(d);

frontend/tests/unit/tests/components/input/transformers/transform-date-test.js

@@ -70,10 +70,11 @@ describe('transformDateValue Directive', function() {
   });
 
   describe('when given invalid date values', function () {
-    shouldBehaveLikeAParser('', undefined);
-    shouldBehaveLikeAParser('invalid', undefined);
-    shouldBehaveLikeAParser('2016-12', undefined);
+    shouldBehaveLikeAParser('', null);
+    shouldBehaveLikeAParser('invalid', null);
+    shouldBehaveLikeAParser('2016-12', null);
     shouldBehaveLikeAFormatter(undefined, '');
+    shouldBehaveLikeAFormatter(null, '');
     shouldBehaveLikeAFormatter('2016-12', '');
   });
 });