Allow to query mapping value when receives MapRequest

Change-Id: Ic7893a397c2c16e219135a31516565d2f1323f50
diff --git a/apps/mappingmanagement/api/src/main/java/org/onosproject/mapping/MappingProviderService.java b/apps/mappingmanagement/api/src/main/java/org/onosproject/mapping/MappingProviderService.java
index 4ff6e29..724f493 100644
--- a/apps/mappingmanagement/api/src/main/java/org/onosproject/mapping/MappingProviderService.java
+++ b/apps/mappingmanagement/api/src/main/java/org/onosproject/mapping/MappingProviderService.java
@@ -33,6 +33,7 @@
 
     /**
      * Signals that a new mapping query has been issued.
+     * If no mapping is found, simply returns null.
      *
      * @param mappingKey a mapping key that is used for query a mapping value
      * @return a mapping value associated with a given mapping key