Don't call listen with empty directory list

Listen interprets an empty list of directories as "watch the current
directory". Since EventedFileUpdateChecker doesn't share these
semantics, we should not initialize listen if we end up with an empty
directory list.

activesupport/lib/active_support/evented_file_update_checker.rb

@@ -108,7 +108,10 @@ module ActiveSupport
     private
       def boot!
         normalize_dirs!
-        Listen.to(*@dtw, &method(:changed)).start
+
+        unless @dtw.empty?
+          Listen.to(*@dtw, &method(:changed)).start
+        end
       end
 
       def shutdown!