OF-1252 Move and tweak null store cache message (#722)

Move the logged exception when `null` keys are allowed for storage to debug leve.

src/java/org/jivesoftware/util/cache/DefaultCache.java

@@ -709,7 +709,7 @@ public class DefaultCache<K, V> implements Cache<K, V> {
             }
         } catch (NullPointerException e) {
             if (allowNull) {
-                Log.error("Ignoring null store in Cache: ", e); // Gives us a trace for debugging.
+                Log.debug("Allowing storage of null within Cache: ", e); // Gives us a trace for debugging.
             } else {
                 throw e;
             }