Handle the missing summary

Prior to this change, if there were no summary provided
in the schema json it displayed nothing, which is a bit
confusing.

This change shows a grayout [none] when the summary is
not included in the json.

gcp/appengine/frontend3/src/styles.scss

@@ -637,6 +637,10 @@ dl.vulnerability-details,
     margin-top: 16px;
   }
 
+  dd.muted {
+    color: $osv-grey-600;
+  }
+
   dd.details {
     overflow-wrap: break-word;
 

gcp/appengine/frontend3/src/templates/vulnerability.html

@@ -63,7 +63,13 @@ <h1 class="title">{{ vulnerability.id }}</h1>
           <dt>Modified</dt>
           <dd>{{ vulnerability.modified }}</dd>
           <dt class="summary">Summary</dt>
-          <dd class="summary">{{ vulnerability.summary }}</dd>
+          <dd class="summary {% if not vulnerability.summary %}muted{% endif %}">
+            {% if vulnerability.summary %}
+            {{ vulnerability.summary }}
+            {% else %}
+            [none]
+            {% endif %}
+          </dd>
           <dt>Details</dt>
           <dd class="details">
             {{ vulnerability.details|markdown|safe -}}