Render problems only when there's a problem list

src/allmydata/web/map-update-status.xhtml

@@ -22,7 +22,7 @@
     <h2>Update Results</h2>
 
     <ul>
-      <li t:render="problems" />
+      <t:transparent t:render="problems" />
       <li>Total: <span t:render="time_total" /></li>
       <ul>
         <li>Initial Queries: <span t:render="time_initial_queries" /></li>

src/allmydata/web/status.py

@@ -931,7 +931,7 @@ class MapupdateStatusElement(Element):
     def problems(self, req, tag):
         problems = self._update_status.problems
         if not problems:
-            return tag("")
+            return tag
         l = tags.ul()
         for peerid in sorted(problems.keys()):
             peerid_s = idlib.shortnodeid_b2a(peerid)