Disable sorttable on tables without rows

Otherwise, the first element of the table body is being fetched which might result in an error for empty tables.

Fixes CODEOCEAN-JZ

app/views/exercises/statistics.html.slim

@@ -61,8 +61,9 @@ h1 = @exercise
   - if !policy(@exercise).detailed_statistics?
     - submissions = submissions.final
   - if submissions.any?
+    - users = symbol.to_s.classify.constantize.where(id: submissions.joins(symbol).group(:user_id).select(:user_id).distinct)
     .table-responsive
-      table.table.table-striped.sortable
+      table.table.table-striped class="#{users.present? ? 'sortable' : ''}"
         thead
           tr
             th.header = t('.user')
@@ -71,7 +72,6 @@ h1 = @exercise
             th.header = t('.runs') if policy(@exercise).detailed_statistics?
             th.header = t('.worktime') if policy(@exercise).detailed_statistics?
         tbody
-          - users = symbol.to_s.classify.constantize.where(id: submissions.joins(symbol).group(:user_id).select(:user_id).distinct)
           - users.each do |user|
             - if user_statistics[user.class.name][user.id] then us = user_statistics[user.class.name][user.id] else us = {"maximum_score" => nil, "runs" => nil}
             - label = "#{user.displayname}"