Fix missing values and prevent value stack lookup
Fix missing values and prevent value stack lookup

It items has missing attributes which siblings do have, fix it by adding
an empty array or null to the item scheme.

Prevents scenarios like:

```yaml
---
_id: 123
title: Some page
items:
  -
    href: /root1
    text: Root 1
  -
    href: /root2
    text: Root 2
breadcrumbs:
    -
      href: /root2/sub1
      title: Subitem 1
      # missing 'items' attribute forces lookup in hight context
      # and resolve to root 'items'
    -
      href: /root2/sub2
      title: Subitem 2
      items:
            -
              href: /root2/sub2/subpage1
              # missing title would result in printing `Subitem2`
            -
              href: /root2/sub2/subpage2
              title: Subpage 2
...
```

attitude/mustache-data-preprocessor