do not add/remove null values

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

@@ -20,7 +20,11 @@ public class LRUCache<K, V> {
     }
 
     public V put(final K key, final V value) {
-        return cache.put(key, value);
+        if (value == null) {
+            return remove(key);
+        } else {
+            return cache.put(key, value);
+        }
     }
 
     public V get(final K key) {