fix: GameService.load() NPE on null bundleFile + ZipService test path

- GameService.load(): null-check bundleFile before resolving path
  (Path.resolve(null) throws NullPointerException)
- ZipServiceTest: pass absolute exePath to createBundle (relativize
  requires both paths to be absolute or both relative)

src/main/java/org/dostalgia/GameService.java

@@ -77,9 +77,12 @@ public class GameService implements GameStore {
         );
         // Detect bundle size
         try {
-            Path bundlePath = gamesDir.resolve(game.getBundleFile());
-            if (Files.exists(bundlePath)) {
-                game.setBundleSize(Files.size(bundlePath));
+            String bf = game.getBundleFile();
+            if (bf != null) {
+                Path bundlePath = gamesDir.resolve(bf);
+                if (Files.exists(bundlePath)) {
+                    game.setBundleSize(Files.size(bundlePath));
+                }
             }
         } catch (IOException ignored) {}
         return game;