Having difficulties with Jekyll / Liquid

Answer 1

Change your if statements to just check if post.previous exists.

{% if post.previous %}
<span class="page-nav-item">
    <a rel="prev" href="{{ post.previous.url }}" title="View {{ post.previous.title }}">&larr; View previous article</a>
</span>
{% endif %}
{% if post.next %}
<span class="page-nav-item">
    <a rel="next" href="{{ post.next.url }}" title="View {{ post.next.title }}">View next article &rarr;</a>
</span>
{% endif %}

Answer 2

I had better luck using page.next/previous

    {% if page.previous %} 
        <a rel="prev" href="{{ page.previous.url }}">&larr; Older</a>
    {% endif %}
    {% if page.next %} 
        <a rel="next" href="{{ page.next.url }}">Newer &rarr;</a>
    {% endif %}