do not add/remove null values

pdb-utils/src/main/java/org/lucares/utils/cache/LRUCache.java

@@ -20,8 +20,12 @@ public class LRUCache<K, V> {
     }
 
     public V put(final K key, final V value) {
+        if (value == null) {
+            return remove(key);
+        } else {
             return cache.put(key, value);
         }
+    }
 
     public V get(final K key) {
         final V result = cache.get(key);