fix: do not ignore missing context on Ash.PlugHelpers.update_context/2 (#2674)

Previously, it was swallowing the case when context was not yet
initialized. Currently, it will just call the callback and set the
context.

Author: smt116

lib/ash/plug_helpers.ex

@@ -244,14 +244,9 @@ if Code.ensure_loaded?(Plug.Conn) do
     @spec update_context(Conn.t(), (nil | map() -> nil | map())) ::
             Conn.t()
     def update_context(conn, callback) do
-      case get_context(conn) do
-        nil ->
-          conn
-
-        context ->
-          conn
-          |> set_context(callback.(context))
-      end
+      get_context(conn)
+      |> callback.()
+      |> set_context()
     end
   end
 else