Clarify Near Cache null values behaviour [CTT-833] [5.5.z]

docs/modules/cluster-performance/pages/near-cache.adoc

@@ -563,6 +563,18 @@ If it is expired, it is evicted and `null` is returned as the value to the calle
 Note that `max-idle-seconds` and `time-to-live-seconds` are configured under the `near-cache` configuration block, as seen in the above
 <<near-cache-configuration-examples, configuration examples>>.
 
+== Caching of null values
+
+When you call `get()` on a map with Near Cache enabled and the requested key does not exist in the underlying data structure, the Near Cache stores the `null` result.
+This means that subsequent `get()` calls for the same non-existent key are served directly from the Near Cache without requiring a network round-trip to the cluster.
+
+This behavior provides a performance benefit when your application frequently checks for keys that may not exist, as it avoids repeated server lookups.
+
+The cached `null` value is subject to the same expiration and invalidation rules as any other Near Cache entry:
+
+* It expires based on `max-idle-seconds` and `time-to-live-seconds` configuration.
+* It is invalidated when an entry with that key is added to the underlying data structure.
+
 == Near Cache Invalidation
 
 Invalidation is the process of removing an entry from the Near Cache when its value is updated or