How do I write an if statement to show Isotope filter button only if the Entry Section has content?
Here's just a variation on the conditional that Victor provided that will do the same thing, might be a bit cleaner if you ever need to check several arrays:

{% if albumCd|merge(singleCd)|length  %}
    &lt;button class="filt btn" type="button" data-filter=".CD"&gt;CD&lt;/button&gt;
{% endif %}