Simplify object and association checks

* check for <object> instead of <object>.nil?
* check for <association>_id? instead of <association>.nil? (avoids
  record loading)

app/views/quantities/reparent.turbo_stream.erb

@@ -5,5 +5,5 @@
   <%= turbo_stream.replace ancestor %>
 <% end %>
 <% @self_and_progenies.each do |q| %>
-  <%= @before.nil? ? turbo_stream.append(:quantities, q) : turbo_stream.before(@before, q) %>
+  <%= @before ? turbo_stream.before(@before, q) : turbo_stream.append(:quantities, q) %>
 <% end %>