Codechange: Use null pointer literal instead of the NULL macro

src/genworld.h

@@ -60,8 +60,8 @@ struct GenWorldInfo {
 	CompanyID lc;          ///< The local_company before generating
 	uint size_x;           ///< X-size of the map
 	uint size_y;           ///< Y-size of the map
-	GWDoneProc *proc;      ///< Proc that is called when done (can be NULL)
-	GWAbortProc *abortp;   ///< Proc that is called when aborting (can be NULL)
+	GWDoneProc *proc;      ///< Proc that is called when done (can be nullptr)
+	GWAbortProc *abortp;   ///< Proc that is called when aborting (can be nullptr)
 	std::thread thread;    ///< The thread we are in (joinable if a thread was created)
 };