compiler: add CollectionsUtil#mapOf() for creating an empty map

Author: zapster

compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/util/CollectionsUtil.java

@@ -67,6 +67,17 @@ public static <K, V> Map<K, V> mapOfEntries(Map.Entry<K, V>... entries) {
         return Collections.unmodifiableMap(newMap);
     }
 
+    /**
+     * Creates an unmodifiable empty map.
+     *
+     * @param <K> the key type
+     * @param <V> the value type
+     * @return an unmodifiable empty map
+     */
+    public static <K, V> Map<K, V> mapOf() {
+        return Collections.emptyMap();
+    }
+
     /**
      * Creates an unmodifiable map with a single entry.
      *