build: fix panic when exporting to tar

Fixes a panic on `docker build -t foo -o - . >/dev/null`

Signed-off-by: Tibor Vass <tibor@docker.com>

api/server/backend/build/backend.go

@@ -91,7 +91,9 @@ func (b *Backend) Build(ctx context.Context, config backend.BuildConfig) (string
 		stdout := config.ProgressWriter.StdoutFormatter
 		fmt.Fprintf(stdout, "Successfully built %s\n", stringid.TruncateID(imageID))
 	}
-	err = tagger.TagImages(image.ID(imageID))
+	if imageID != "" {
+		err = tagger.TagImages(image.ID(imageID))
+	}
 	return imageID, err
 }