I already added this statement to javadoc of `PartitioningStrategy`;

"@return partition key object or null to fallback to default partition calculation"

Also changed code to deal with returning the original key instead of null. Like;

````java
Object pk = strategy.getPartitionKey(obj);
if (pk != null && pk != obj) {
    final Data partitionKey = toData(pk, EMPTY_PARTITIONING_STRATEGY);
    data.partitionHash = (partitionKey == null) ? -1 : partitionKey.getPartitionHash();
}
````