Liquid filter collection where not null

In my front matter for some pages (not all) I have:

---
top-navigation:
    order: 2
---

Using liquid I want to filter all site pages which have a top-navigation object and sort by top-navigation.order.

I'm trying sort:'top-navigation.order' but that's throwing an exception of undefined method [] for nil:NilClass. I tried where:"top-navigation", true but it's not equating truthy values.

How can I filter for pages that have top-navigation and then sort?

Two steps:

1. Create an array with pages that contains the top-navigation key.

   We create an empty array and then push the items that have the key.

   {% assign navposts = ''|split:''%}
   {% for post in site.posts %}
   {% if post.top-navigation %}
   {% assign navposts = navposts|push:post%}
   {% endif %}
   {% endfor %}
   

2. Sort the above array by top-navigation.order

   {% assign navposts = navposts|sort: "top-navigation.order"%}
   

Printing results:

{% for post in navposts %}
<br>{{ post.title }} - {{post.top-navigation}}
{% endfor %}

For pages use site.pages.