Fix data transformers to remove a nil value of dbi_server for syncing.

Map.put_new don't update nil value, because the key exists.

lib/decentralised_book_index/sync/data_transformers/author_transformer.ex

@@ -20,6 +20,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.AuthorTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end

lib/decentralised_book_index/sync/data_transformers/book_transformer.ex

@@ -26,6 +26,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.BookTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end

lib/decentralised_book_index/sync/data_transformers/dbi_server_transformer.ex

@@ -18,6 +18,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.DBIServerTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end

lib/decentralised_book_index/sync/data_transformers/publisher_transformer.ex

@@ -17,6 +17,10 @@ defmodule DecentralisedBookIndex.Sync.DataTransformers.PublisherTransformer do
         dbi_server_id: get_in(json_body, ["attributes", "dbi_server_id"])
       }
 
-    {:ok, attrs}
+    if is_nil(attrs[:dbi_server_id]) do
+      {:ok, attrs |> Map.delete(:dbi_server_id)}
+    else
+      {:ok, attrs}
+    end
   end
 end