feat(talk): do not require `event`

Close #1397

layouts/talk/single.html

@@ -27,6 +27,7 @@
     </div>
     <div class="d-md-none space-below"></div>
 
+    {{ if .Params.event }}
     <div class="row">
       <div class="col-md-1"></div>
       <div class="col-md-10">
@@ -37,11 +38,12 @@
             {{ .Params.event | markdownify }}
             {{ if .Params.event_url }}</a>{{ end }}
           </div>
-        </div>
+        </div> 
       </div>
       <div class="col-md-1"></div>
     </div>
     <div class="d-md-none space-below"></div>
+    {{ end }}
 
     {{ if .Params.location }}
     <div class="row">