expandable inserts <ul> tags when list is small

[GuillermoFidalgo]

I wanted to reuse the code

{% include get_all_events.html %}
{% assign sorted_events = all_events | sort: 'startdate' | reverse %}
{% assign training_events = "" | split: "," %}
{% for event in sorted_events %}
{% if event.labels %}
{% if event.labels contains "outreach" %}
{% assign training_events = training_events | push: event %}
{% endif %}
{% endif %}
{% endfor %}

{% expandable training_events 7 %}
{% include print_event.html students=1 event=expandable %}
{% endexpandable %}

on the newly renamed "outreach" page proposed in #2019 but I noticed when testing that if the interger argument for this line {% expandable training_events 7 %} is greater than the number of elements available (i.e we have only 2 events in the array training_events but I ask for 7 before the "Show More" box appears) then it renders
some unordered lists tags on the page.

This is not a big deal as eventually the array will contain more events, or I could simply make the number smaller for now. But it would nice for those tags to not render when we have few events to list.