fix(datepicker): null model with timezone throws exception

- verify that the model value is not null or undefined before accessing
  `.getTimezoneOffset()`

Fixes #12025

Author: Splaktar

src/components/datepicker/js/datepickerDirective.js

@@ -992,7 +992,7 @@
     var timezone = this.$mdUtil.getModelOption(this.ngModelCtrl, 'timezone');
     // Using the timezone when the offset is negative (GMT+X) causes the previous day to be
     // set as the model value here. This check avoids that.
-    if (timezone == null || value.getTimezoneOffset() < 0) {
+    if (timezone == null || value == null || value.getTimezoneOffset() < 0) {
       this.ngModelCtrl.$setViewValue(this.ngDateFilter(value, 'yyyy-MM-dd'), 'default');
     } else {
       this.ngModelCtrl.$setViewValue(this.ngDateFilter(value, 'yyyy-MM-dd', timezone), 'default');
@@ -1014,7 +1014,7 @@
     }
     // Using the timezone when the offset is negative (GMT+X) causes the previous day to be
     // used here. This check avoids that.
-    if (timezone == null || value.getTimezoneOffset() < 0) {
+    if (timezone == null || value == null || value.getTimezoneOffset() < 0) {
       this.inputElement.value = this.locale.formatDate(value);
     } else {
       this.inputElement.value = this.locale.formatDate(value, timezone);

src/components/datepicker/js/datepickerDirective.spec.js

@@ -139,13 +139,21 @@ describe('md-datepicker', function() {
     expect(ngElement.attr('type')).toBe('date');
   });
 
+  it('should handle an initial ng-model that is null when timezone is specified', function() {
+    pageScope.modelOptions = {timezone: 'UTC'};
+    pageScope.myDate = null;
+    createDatepickerInstance(
+      '<md-datepicker ng-model="myDate" ng-model-options="modelOptions"></md-datepicker>');
+  });
+
   it('should pass the timezone to the formatting function', function() {
     spyOn(controller.locale, 'formatDate');
+    pageScope.modelOptions = {timezone: 'UTC'};
 
-    createDatepickerInstance('<md-datepicker ng-model="myDate" ' +
-      'ng-model-options="{ timezone: \'utc\' }"></md-datepicker>');
+    createDatepickerInstance(
+      '<md-datepicker ng-model="myDate" ng-model-options="modelOptions"></md-datepicker>');
 
-    expect(controller.locale.formatDate).toHaveBeenCalledWith(pageScope.myDate, 'utc');
+    expect(controller.locale.formatDate).toHaveBeenCalledWith(pageScope.myDate, 'UTC');
   });
 
   it('should allow for the locale to be overwritten on a specific element', function() {