fix(datepicker): fallback to current date

- Fallback to current date if model not present

Closes #5418
Fixes #5417

Author: trickpattyFH20

Diff for: src/datepicker/datepicker.js

@@ -217,9 +217,7 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
     ngModelCtrl = ngModelCtrl_;
     ngModelOptions = ngModelCtrl_.$options || datepickerConfig.ngModelOptions;
 
-    if (ngModelCtrl.$modelValue) {
-      this.activeDate = ngModelCtrl.$modelValue;
-    }
+    this.activeDate = ngModelCtrl.$modelValue || new Date();
 
     ngModelCtrl.$render = function() {
       self.render();