Fix HttpHeaders.getAcceptableLanguages() on empty headers

`jakarta.ws.rs.core.HttpHeaders.getAcceptableLanguages` requires
that a wildcard locale is returned, if no acceptable language is
specified. The previous behavior returned an empty list.

Closes #44253

independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/HeaderUtil.java

@@ -32,6 +32,7 @@
  */
 @SuppressWarnings({ "rawtypes", "unchecked" })
 public class HeaderUtil {
+    private static final List<Locale> LANGUAGE_WILDCARD = List.of(Locale.ROOT);
 
     private static final ClassValue<RuntimeDelegate.HeaderDelegate<?>> HEADER_DELEGATE_CACHE = new ClassValue<>() {
         @Override
@@ -293,7 +294,7 @@ public static List<MediaType> getAcceptableMediaTypes(MultivaluedMap<String, ? e
     public static List<Locale> getAcceptableLanguages(MultivaluedMap<String, ? extends Object> headers) {
         List<?> accepts = headers.get(HttpHeaders.ACCEPT_LANGUAGE);
         if (accepts == null || accepts.isEmpty())
-            return Collections.emptyList();
+            return LANGUAGE_WILDCARD;
         List<WeightedLanguage> languages = new ArrayList<WeightedLanguage>();
         for (Object obj : accepts) {
             if (obj instanceof Locale) {