[rails6][MiqServer] Use proper ActionView constructor

This was changed in Rails 6:

rails/rails@e17fe52

And using `.empty` is good enough for our case since we are just trying
to access the helper method, `.number_to_human_size`, outside the
context of the views.

app/models/miq_server/environment_management.rb

@@ -74,7 +74,7 @@ def check_disk_usage(disks)
                          end
 
       next unless disk_usage_event
-      msg = "Filesystem: #{disk[:filesystem]} (#{disk[:type]}) on #{disk[:mount_point]} is #{disk[:used_bytes_percent]}% full with #{ActionView::Base.new.number_to_human_size(disk[:available_bytes])} free."
+      msg = "Filesystem: #{disk[:filesystem]} (#{disk[:type]}) on #{disk[:mount_point]} is #{disk[:used_bytes_percent]}% full with #{ActionView::Base.empty.number_to_human_size(disk[:available_bytes])} free."
       MiqEvent.raise_evm_event_queue(self, disk_usage_event, :event_details => msg)
     end
   end