Provides null checks for mapping action and address codecs

Change-Id: I210c3516fd335fe6c551e7ed933ad304516bd8dd
diff --git a/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingActionCodec.java b/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingActionCodec.java
index 169d819..0ffd088 100644
--- a/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingActionCodec.java
+++ b/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingActionCodec.java
@@ -43,6 +43,10 @@
 
     @Override
     public MappingAction decode(ObjectNode json, CodecContext context) {
+        if (json == null || !json.isObject()) {
+            return null;
+        }
+
         DecodeMappingActionCodecHelper decoder =
                 new DecodeMappingActionCodecHelper(json);
         return decoder.decode();
diff --git a/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingAddressCodec.java b/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingAddressCodec.java
index 0458cf5..0c6d7d9 100644
--- a/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingAddressCodec.java
+++ b/apps/mappingmanagement/web/src/main/java/org/onosproject/mapping/web/codec/MappingAddressCodec.java
@@ -46,6 +46,10 @@
 
     @Override
     public MappingAddress decode(ObjectNode json, CodecContext context) {
+        if (json == null || !json.isObject()) {
+            return null;
+        }
+
         DecodeMappingAddressCodecHelper decoder =
                             new DecodeMappingAddressCodecHelper(json);
         return decoder.decode();