Update to allow a Book to have no Publisher.

lib/decentralised_book_index/metadata/book.ex

@@ -373,7 +373,6 @@ defmodule DecentralisedBookIndex.Metadata.Book do
     end
 
     belongs_to :publisher, Metadata.Publisher do
-      allow_nil? false
       public? true
     end
 

lib/decentralised_book_index_web/live/book_live/show.ex

@@ -64,12 +64,14 @@ defmodule DecentralisedBookIndexWeb.BookLive.Show do
         <dt class="mb-2 font-semibold leading-none text-gray-900 dark:text-white">Description</dt>
         <dd class="mb-4 font-light text-gray-500 sm:mb-5 dark:text-gray-400">{@book.description}</dd>
       </dl>
-      <dl>
-        <dt class="mb-2 font-semibold leading-none text-gray-900 dark:text-white">Publisher</dt>
-        <dd class="mb-4 font-light text-gray-500 sm:mb-5 dark:text-gray-400">
-          {@book.publisher.name}
-        </dd>
-      </dl>
+      <%= if not is_nil(@book.publisher) do %>
+        <dl>
+          <dt class="mb-2 font-semibold leading-none text-gray-900 dark:text-white">Publisher</dt>
+          <dd class="mb-4 font-light text-gray-500 sm:mb-5 dark:text-gray-400">
+            {@book.publisher.name}
+          </dd>
+        </dl>
+      <% end %>
       <dl>
         <dt class="mb-2 font-semibold leading-none text-gray-900 dark:text-white">Published</dt>
         <dd class="mb-4 font-light text-gray-500 sm:mb-5 dark:text-gray-400">{@book.published}</dd>