Allow select merging maps with all nil values (#4601)

Author: greg-rychlewski

Diff for: integration_test/cases/repo.exs

@@ -1538,14 +1538,14 @@ defmodule Ecto.Integration.RepoTest do
                |> select_merge([_l, p], map(p, ^~w(title posted)a))
                |> TestRepo.all()
 
-      # left join record is not present
-      assert [%{url: "Q", title: "Z", posted: nil}] =
+      # left join record is not present, we consider it the same as being present with nils
+      assert [%{url: "Q", title: nil, posted: nil}] =
                Permalink
                |> join(:left, [l], p in Post, on: l.post_id == p.id and p.public == true)
                |> select([l, p], merge(l, map(p, ^~w(title posted)a)))
                |> TestRepo.all()
 
-      assert [%{url: "Q", title: "Z", posted: nil}] =
+      assert [%{url: "Q", title: nil, posted: nil}] =
                Permalink
                |> join(:left, [l], p in Post, on: l.post_id == p.id and p.public == true)
                |> select_merge([_l, p], map(p, ^~w(title posted)a))

Diff for: lib/ecto/repo/queryable.ex

@@ -310,7 +310,7 @@ defmodule Ecto.Repo.Queryable do
 
   defp process(row, {:merge, left, right}, from, adapter) do
     {left, row} = process(row, left, from, adapter)
-    {right, row} = process(row, right, from, adapter)
+    {right, row} = process_merge(row, right, from, adapter)
 
     data =
       case {left, right} do
@@ -427,6 +427,15 @@ defmodule Ecto.Repo.Queryable do
     end)
   end
 
+  defp process_merge(row, {:source, {source, schema}, prefix, types}, _from, adapter) do
+    struct = Ecto.Schema.Loader.load_struct(schema, prefix, source)
+    struct_load!(types, row, [], false, struct, adapter)
+  end
+
+  defp process_merge(row, process, from, adapter) do
+    process(row, process, from, adapter)
+  end
+
   @compile {:inline, load!: 5}
   defp load!(type, value, field, struct, adapter) do
     case Ecto.Type.adapter_load(adapter, type, value) do