Fix: Only if it would be non-null

projects/tentacles/src/python/tentacles/templates/jobs_history.html.j2

@@ -20,10 +20,12 @@
       <label for="filename">File</label>
       <span name="filename">{{ctx.db.fetch_file(ctx.uid, job.file_id).filename or "it's a secret"}}</span>
     </div>
+    {% if job.finished_at and job.started_at %}
     <div class="job-runtime u-flex">
       <label for="runtime">Runtime</label>
       <span name="Runtime">{{ (datetime.fromisoformat(job.finished_at) - datetime.fromisoformat(job.started_at)) }}</span>
     </div>
+    {% endif %}
     </div>
   <div class="controls u-flex u-ml-auto">
     {% if ctx.uid %}