Fix zero division error when a day has no sleep entries.

dashboard/templates/cards/sleep_day.html

@@ -14,9 +14,11 @@
 {% endblock %}
 
 {% block content %}
-    <div class="text-muted">
-        Average sleep duration: <strong>{{ average|duration_string }}</strong>.
-    </div>
+    {% if total %}
+        <div class="text-muted">
+            Average sleep duration: <strong>{{ average|duration_string }}</strong>.
+        </div>
+    {% endif %}
 {% endblock %}
 
 {% block footer %}{{ count }} sleep entries{% endblock %}

dashboard/templatetags/dashboard.py

@@ -121,8 +121,10 @@ def card_sleep_day(child, date=None):
 
         total += end - start
 
-    return {
-        'total': total,
-        'count': len(instances),
-        'average': total/len(instances)
-    }
+    count = len(instances)
+    if count > 0:
+        average = total/count
+    else:
+        average = 0
+
+    return {'total': total, 'count': count, 'average': average}