ldd: treat nil pathname as self

This is a helpful shortcut for examining a test program's ldd output.

Signed-off-by: Ophestra <cat@gensokyo.uk>

ldd/exec_test.go

@@ -34,7 +34,7 @@ func TestExec(t *testing.T) {
 	t.Run("success", func(t *testing.T) {
 		msg := message.New(nil)
 		msg.GetLogger().SetPrefix("check: ")
-		if entries, err := ldd.Resolve(t.Context(), nil, check.MustAbs(container.MustExecutable(msg))); err != nil {
+		if entries, err := ldd.Resolve(t.Context(), nil, nil); err != nil {
 			t.Fatalf("Exec: error = %v", err)
 		} else if testing.Verbose() {
 			// result cannot be measured here as build information is not known