Fix segfault with describe_topics and flaky connection (#1692)

If you call describe_topics on a flaky connection, sometimes the
admin client reply has the host set to a null pointer. When this
occurs, instead of segfaulting, report the host as None.

src/confluent_kafka/src/confluent_kafka.c

@@ -1620,7 +1620,10 @@ PyObject *c_Node_to_py(const rd_kafka_Node_t *c_node) {
 
         cfl_PyDict_SetInt(kwargs, "id", rd_kafka_Node_id(c_node));
         cfl_PyDict_SetInt(kwargs, "port", rd_kafka_Node_port(c_node));
-        cfl_PyDict_SetString(kwargs, "host", rd_kafka_Node_host(c_node));
+        if (rd_kafka_Node_host(c_node))
+                cfl_PyDict_SetString(kwargs, "host", rd_kafka_Node_host(c_node));
+        else
+                PyDict_SetItemString(kwargs, "host", Py_None);
         if((rack = rd_kafka_Node_rack(c_node)))
                 cfl_PyDict_SetString(kwargs, "rack", rack);