refactor: use nullptr for null pointer literals

- replace `0` null pointer literals (e.g. `void* hint = 0`) with `nullptr`
- clang-tidy modernize-use-nullptr
  https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-nullptr.html

fairmq/zeromq/Message.h

@@ -107,7 +107,11 @@ class Message final : public fair::mq::Message
         }
     }
 
-    Message(UnmanagedRegionPtr& region, void* data, const size_t size, void* hint = 0, fair::mq::TransportFactory* factory = nullptr)
+    Message(UnmanagedRegionPtr& region,
+            void* data,
+            const size_t size,
+            void* hint = nullptr,
+            fair::mq::TransportFactory* factory = nullptr)
         : fair::mq::Message(factory)
         , fMsg(std::make_unique<zmq_msg_t>())
     {

fairmq/zeromq/TransportFactory.h

@@ -73,7 +73,10 @@ class TransportFactory final : public fair::mq::TransportFactory
         return std::make_unique<Message>(data, size, ffn, hint, this);
     }
 
-    MessagePtr CreateMessage(UnmanagedRegionPtr& region, void* data, size_t size, void* hint = 0) override
+    MessagePtr CreateMessage(UnmanagedRegionPtr& region,
+                             void* data,
+                             size_t size,
+                             void* hint = nullptr) override
     {
         return std::make_unique<Message>(region, data, size, hint, this);
     }