fix(datepicker): pass through null

- When `minDate` or `maxDate` is `null` in the popup, pass through value to inline datepicker

src/datepicker/datepicker.js

@@ -685,10 +685,18 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
 
         scope.$parent.$watch(getAttribute, function(value) {
           if (key === 'minDate' || key === 'maxDate') {
-            cache[key] = angular.isDate(value) ? dateParser.fromTimezone(new Date(value), ngModelOptions.timezone) : new Date(dateFilter(value, 'medium'));
+            if (value === null) {
+              cache[key] = null;
+            } else if (angular.isDate(value)) {
+              cache[key] = dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
+            } else {
+              cache[key] = new Date(dateFilter(value, 'medium'));
+            }
+
+            scope.watchData[key] = value === null ? null : cache[key];
+          } else {
+            scope.watchData[key] = dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
           }
-
-          scope.watchData[key] = cache[key] || dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
         });
 
         datepickerEl.attr(cameltoDash(key), 'watchData.' + key);