Add warning for messages

app/views/contests/show.html.slim

@@ -50,10 +50,10 @@
       .row.mt-5
         .col
           h4 = t("messages.plural").capitalize
-      - if !@puzzles.size
+      - if @puzzles.size == 0
         .row
           .col.alert.alert-danger
-            | You first need to add a puzzle before converting messages to completions
+            = t("messages.warning")
       table.table.table-striped.table-hover
         thead
           tr
@@ -82,7 +82,7 @@
               td
                 = message.text
               td
-                - if @puzzles.size
+                - if @puzzles.size > 0
                   a.btn.btn-sm.btn-secondary href=contest_message_convert_path(@contest, message) style="white-space: nowrap;"
                     = t("helpers.buttons.add_completion")
                 - else