Adjust behaviour of changed?/2 for assocs

It now returns true whenever `fetch_change? != :error`. Previously, it returned
false when assoc was set to existing struct (as `changeset.changes == %{}`) and
errored when set to nil (due to `nil.changes != %{}` check in `relation_changed?`.

Fixes #4543.

lib/ecto/changeset.ex

@@ -538,12 +538,12 @@ defmodule Ecto.Changeset do
         case fetch_change(changeset, field) do
           {:ok, new_value} ->
             case type do
-              {tag, relation} when tag in @relations ->
+              {tag, _relation} when tag in @relations ->
                 if opts != [] do
                   raise ArgumentError, "invalid options for #{tag} field"
                 end
 
-                relation_changed?(relation.cardinality, new_value)
+                true
 
               _ ->
                 Enum.all?(opts, fn
@@ -567,14 +567,6 @@ defmodule Ecto.Changeset do
     end
   end
 
-  defp relation_changed?(:one, changeset) do
-    changeset.action != :update or changeset.changes != %{}
-  end
-
-  defp relation_changed?(:many, changesets) do
-    Enum.any?(changesets, &relation_changed?(:one, &1))
-  end
-
   @doc """
   Returns the default empty values used by `Ecto.Changeset`.