URL param with type date and default value null breaks

This is a (check one box):

- [X] Bug Report
- [ ] Feature Request
- [ ] General Query

My version of UI-Router is: (type version)

1.0.27

# Bug Report

#### Current Behavior:

A URL param declaration with `type: 'date'` and default `value: null` throws:
`Cannot read property 'getFullYear' of null`

#### Expected Behavior:

It should work and set null as default value for this param

#### Link to Plunker or stackblitz that reproduces the issue:

( if you want a response to your issue, provide a way to reproduce it )
( http://bit.ly/UIR-Plunk1 )
( https://stackblitz.com/edit/ui-router-angularjs )