Display elements only if content is defined in frontpage templates

_includes/frontpage/event-card.html

 {% assign event = include.event %}
 {% assign location = site.data.hifis_organizations[event.location.campus] %}
+{% if event %}
 <div class="frontpage-card">
     <div class="card-body">
+        {% if event.start.date %}
         <span class="badge badge-danger">
             <i class="fas fa-calendar-day" aria-hidden="true"></i> {{ event.start.date | date_to_string }}
         </span>
+        {% endif %}
+        {% if event.type %}
         <span class="badge badge-secondary">
             <i class="fas fa-flag" aria-hidden="true"></i> {{ event.type | capitalize }}
         </span>
+        {% endif %}
+        {% if location.city %}
         <span class="badge badge-primary">
             <i class="fas fa-location-arrow" aria-hidden="true"></i> {{ location.city }}
         </span>
+        {% endif %}
+        {% if event.title %}
         <h3 class="card-title">
             <a href="{{ event.url | relative_url }}">{{ event.title }}</a>
         </h3>
-        <p class="card-text">{{ event.excerpt | strip_html | slice: 0, 150 }}</p>
+        {% else %}
+        <!--
+            WARNING: event has no title defined
+        -->
+        {% endif %}
+        {% if event.excerpt %}
+        <p class="card-text">{{ event.excerpt | strip_html | slice: 0, 150 }}</p>        
+        {% endif %}
         <a href="{{ event.url | relative_url }}" class="btn btn-primary">
             <i class="fas fa-info-circle" aria-hidden="true"></i>&nbsp; More
         </a>
     </div>
 </div>
+{% else %}
+    <!--
+        WARNING: frontpage/event-card.html was included,
+        but no event object was defined
+    -->
+{% endif %}

_includes/frontpage/image-card.html

@@ -2,9 +2,20 @@
 {% assign url = include.url %}
 {% assign caption = include.caption %}
 <div class="frontpage-image-card">
+    {% if image_path %}
     <img src="{{ image_path | relative_url }}" class="card-img" alt="{{ caption }}">
+    {% else %}
+        <!--
+            WARNING: frontpage/image-card.html was included but now image was
+                     defined
+        -->
+    {% endif %}
     <div class="card-img-overlay">
+        {% if caption %}
         <h3 class="card-title">{{ caption }}</h3>
+        {% endif %}
+        {% if url %}
         <a href="{{ url | relative_url }}" class="btn btn-outline-light">Learn more</a>
+        {% endif %}
     </div>
 </div>

_includes/frontpage/post-card.html

 {% assign post = include.post %}
 <div class="frontpage-card">
     <div class="card-body">
+        {% if post.title %}
         <a href="{{ post.url | relative_url }}">
             <h3 class="card-title">{{ post.title }}</h3>
         </a>
+        {% else %}
+        <!--
+            WARNING: frontpage/post-card.html was include, but no post title
+                     is defined.
+        -->
+        {% endif %}
+        {% if post.date %}
         <div>
             <i class="fas fa-calendar-day" aria-hidden="true"></i> {{ post.date | date_to_string }}
         </div>
+        {% endif %}
         <div>
         {% for category in post.categories %}
             <span class="badge badge-secondary" aria-hidden="true">{{ category | upcase }}</span>