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/shmem/Message.h

@@ -77,7 +77,12 @@ class Message final : public fair::mq::Message
         fManager.IncrementMsgCounter();
     }
 
-    Message(Manager& manager, UnmanagedRegionPtr& region, void* data, const size_t size, void* hint = 0, fair::mq::TransportFactory* factory = nullptr)
+    Message(Manager& manager,
+            UnmanagedRegionPtr& region,
+            void* data,
+            const size_t size,
+            void* hint = nullptr,
+            fair::mq::TransportFactory* factory = nullptr)
         : fair::mq::Message(factory)
         , fManager(manager)
         , fLocalPtr(static_cast<char*>(data))