Merge pull request #74 from shweta-mane/master

In JSON extractor allow null value for pagination session stop condition

cdi-core/src/main/java/com/linkedin/cdi/extractor/JsonExtractor.java

@@ -437,7 +437,7 @@ private String retrieveSessionKeyValue(JsonElement input) {
       if (e.getAsJsonObject().has(member)) {
         e = e.getAsJsonObject().get(member);
         if (!members.hasNext()) {
-          return e.getAsString();
+          return e.isJsonNull() ? "null" : e.getAsString();
         }
       }
     }