fix(datepicker): default to current date

- When model is empty, default to current date

Closes angular-ui#5395
Fixes angular-ui#5190

src/datepicker/datepicker.js

@@ -813,6 +813,7 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
 
             scope.watchData[key] = value === null ? null : cache[key];
           } else {
+            var date = value ? new Date(value) : new Date();
             scope.watchData[key] = dateParser.fromTimezone(new Date(value), ngModelOptions.timezone);
           }
         }));