fix: avoid empty error on upserts with :nothing

docs: add some small docs for custom extensions

Author: zachdaniel

lib/data_layer.ex

@@ -1219,7 +1219,7 @@ defmodule AshPostgres.DataLayer do
                  upsert_set
                ) do
             :empty ->
-              :nothing
+              {:replace, options[:upsert_keys] || Ash.Resource.Info.primary_key(resource)}
 
             {:ok, query} ->
               query

lib/repo.ex

@@ -12,6 +12,9 @@ defmodule AshPostgres.Repo do
 
   To configure your list of installed extensions, define `installed_extensions/0`
 
+  Extensions can be a string, representing a standard postgres extension, or a module that implements `AshPostgres.CustomExtension`.
+  That custom extension will be called to generate migrations that serve a specific purpose.
+
   Extensions that are relevant to ash_postgres:
 
   * "ash-functions" - This isn't really an extension, but it expresses that certain functions
@@ -23,7 +26,7 @@ defmodule AshPostgres.Repo do
 
   ```
   def installed_extensions() do
-    ["pg_trgm", "uuid-ossp", "vector"]
+    ["pg_trgm", "uuid-ossp", "vector", YourCustomExtension]
   end
   ```