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/units/new.turbo_stream.erb

@@ -8,11 +8,11 @@
 
 <%= turbo_stream.append :unit_form do %>
   <%- tabular_form_with model: @unit, html: {id: ids[:form_tag]} do |form| %>
-    <%= form.hidden_field :base_id unless @unit.base.nil? %>
+    <%= form.hidden_field :base_id if @unit.base_id? %>
   <% end %>
 <% end %>
 
-<% if @unit.base %>
+<% if @unit.base_id? %>
   <%= turbo_stream.remove ids[:row] %>
   <%= turbo_stream.after @unit.base, partial: 'form', locals: ids %>
 <% else %>