fix(datepicker): default to current date

- When model is empty, default to current date

Closes #5395
Fixes #5190

Author: wesleycho

src/datepicker/datepicker.js

@@ -813,7 +813,8 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
 
             scope.watchData[key] = value === null ? null : cache[key];
           } else {
-            scope.watchData[key] = dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
+            var date = value ? new Date(value) : new Date();
+            scope.watchData[key] = dateParser.fromTimezone(date, ngModelOptions.timezone);
           }
         }));