Preprocessing of Event checks for reference improperly

Problem/Motivation

If an event of a type that can reference a meeting, but the event does not have one, WSOD occurs.

Steps to reproduce

Given event type meeting_calendar_event with an entity_reference type field field_meeting (referencing a live_meeting content type in my example). This field_meeting is not required.
Create a meeting_calendar_event, but do not provide a reference.
Attempt to view the new event (wsod)

Proposed resolution

opigno_calendar_event.module, in template_preprocess_opigno_calendar_event, line 267 checks for "empty", but there is always a non-empty in the reference field. Using $event->field_meeting->isEmpty() works here.

Remaining tasks

submit patch.

User interface changes

n/a

API changes

n/a

Data model changes

n/a