Fix: fix pglogrepl.SendStandbyCopyDone

Change the return value '*CopyDoneResult' to struct to avoid NPE when receive a DataRow message.

pglogrepl.go

@@ -671,7 +671,7 @@ type CopyDoneResult struct {
 
 // SendStandbyCopyDone sends a StandbyCopyDone to the PostgreSQL server
 // to confirm ending the copy-both mode.
-func SendStandbyCopyDone(_ context.Context, conn *pgconn.PgConn) (cdr *CopyDoneResult, err error) {
+func SendStandbyCopyDone(_ context.Context, conn *pgconn.PgConn) (cdr CopyDoneResult, err error) {
 	// I am suspicious that this is wildly wrong, but I'm pretty sure the previous
 	// code was wildly wrong too -- wttw <steve@blighty.com>
 	conn.Frontend().Send(&pgproto3.CopyDone{})

pglogrepl_test.go

@@ -324,7 +324,7 @@ drop table mytable;
 
 	copyDoneResult, err := pglogrepl.SendStandbyCopyDone(ctx, conn)
 	require.NoError(t, err)
-	assert.Nil(t, copyDoneResult)
+	assert.Equal(t, copyDoneResult, pglogrepl.CopyDoneResult{})
 }
 
 func TestBaseBackup(t *testing.T) {