Fix nil comparision

lib/helpers/nav.rb

@@ -103,7 +103,7 @@ module Versioned
       # TODO(ts): Refactor and think about linking directly to the page of the same version.
       first = items
         .find { |i| i.path.start_with?(v[:items_root]) }
-        .children.sort_by { |c| c[:sort_rank] }.first
+        .children.sort_by { |c| c[:sort_rank] || 0 }.first
       %(<option value="#{first.path}" #{selected}>#{v[:name]}</option>)
     end
     classes = active ? 'active' : ''