Fixes NPEs in TranscodingAsyncConsistentMap deserialization code paths

Change-Id: I7898a08d0b999fd6c84ead616856f04bc9765ed5
diff --git a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/TranscodingAsyncConsistentMap.java b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/TranscodingAsyncConsistentMap.java
index c6dd578..a1fe73f 100644
--- a/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/TranscodingAsyncConsistentMap.java
+++ b/core/store/primitives/src/main/java/org/onosproject/store/primitives/impl/TranscodingAsyncConsistentMap.java
@@ -60,9 +60,9 @@
                                    Function<V2, V1> valueDecoder) {
         this.backingMap = backingMap;
         this.keyEncoder = k -> k == null ? null : keyEncoder.apply(k);
-        this.keyDecoder = keyDecoder;
+        this.keyDecoder = k -> k == null ? null : keyDecoder.apply(k);
         this.valueEncoder = v -> v == null ? null : valueEncoder.apply(v);
-        this.valueDecoder = valueDecoder;
+        this.valueDecoder = v -> v == null ? null : valueDecoder.apply(v);
         this.versionedValueTransform = v -> v == null ? null : v.map(valueDecoder);
     }