make JsonNodeClaim always return false for isNull invocation

Author: lbalmaceda

lib/src/main/java/com/auth0/jwt/impl/JsonNodeClaim.java

@@ -102,7 +102,7 @@ public <T> T as(Class<T> tClazz) throws JWTDecodeException {
 
     @Override
     public boolean isNull() {
-        return !(data.isObject() || data.isArray() || data.canConvertToLong() || data.isTextual() || data.isNumber() || data.isBoolean());
+        return false;
     }
 
     /**

lib/src/main/java/com/auth0/jwt/interfaces/Claim.java

@@ -10,6 +10,11 @@
  */
 public interface Claim {
 
+    /**
+     * Whether this Claim has a null value or not.
+     *
+     * @return whether this Claim has a null value or not.
+     */
     boolean isNull();
 
     /**