Fix for null PyObject Pointers at test python wrapper factory (#376)
Fix for null PyObject Pointers at test python wrapper factory (#376)

* Fixes the following incorrect reference decrement crash seen at some builds:

```
Program terminated with signal 11, Segmentation fault.
#0  0x00000000011495ad in hazelcast::client::test::HazelcastServerFactory::shutdownServer (this=0x2cc88f8, member=...) at <https://hazelcast-l337.ci.cloudbees.com/job/cpp-linux-nightly-64-SHARED-Debug/ws/hazelcast/test/src/HazelcastServerFactory.cpp>:192
192                         Py_DECREF(resultObj);

```

Also, better handling of null Python object references.

hazelcast/hazelcast-cpp-client,hazelcast/hazelcast-cpp-client,hazelcast/hazelcast-cpp-client,hazelcast/hazelcast-cpp-client