Fix empty string passed to getElementById error

The cause is in bootstrap.js

```javascript
  function getParent($this) {
    var selector = $this.attr('data-target')

    if (!selector) {
      selector = $this.attr('href')
      selector = selector && /#/.test(selector) && selector.replace(/.*(?=#[^\s]*$)/, '') //strip for ie7
    }

    var $parent = selector && $(selector)

    return $parent && $parent.length ? $parent : $this.parent()
  }
```

Since `data-target` is undefined, the selection is taken from `href` which value is `#`. The `#` is then feed into `$()` which take it as an id selector, strip the `#` leaving only an empty string which get passed to `getElementById`.