Agenda box needs both Start Date and End Date in order to display on of them

Summary

For some reason the Agenda Box pattern requires both Start Date and End Date in order to display one of them.

Steps to reproduce

1. Create a display that uses Agenda Box pattern
2. Add only Start Date without End Date
3. Save and notice that Start Date is not displayed

Possible fixes

In agenda-box.html.twig the following code defines start_date:

        {% if start_date | render and end_date | render %}
            <div class="agenda-box-date-wrapper" {{ attributes_textcolor }}>
                <span class="agenda-box-week-day">{{ start_date|format_date_field('custom', 'l') }}</span>
                <div class="agenda-box-event-dates">
                    <span class="agenda-box-event-dates-1">{{ start_date|format_date_field('custom', 'j') }}</span> <span class="agenda-box-event-dates-2">{{ start_date|format_date_field('custom', 'M') }}</span><span class="agenda-box-event-dates-3">/{{ start_date|format_date_field('custom', 'y') }}</span>
                </div>
                <div class="agenda-box-event-time">
          <span class="agenda-box-start-time">
            {{ start_date|format_date_field('custom', 'H:i') }}
              {% if start_date | format_date_field('custom', 'j M') == end_date | format_date_field('custom', 'j M') %}
                  -
                  {{ end_date|format_date_field('custom', 'H:i') }}
              {% endif %}
          </span>
                    {% if timezone | render %}
                        <span class="agenda-box-time-zone">({{ timezone }})</span>
                    {% endif %}
                    <br/>
                    {% if start_date | format_date_field('custom', 'j M') != end_date | format_date_field('custom', 'j M') %}
                        <span>
              <b>{{ 'Ends:' | t | upper }}</b>
            </span>
                        <b>{{ end_date }}</b>
                    {% endif %}
                </div>
            </div>
        {% endif %}

As you can see in order to render the start_date, it requires the end_date to be present. Change it.