Convert NULL to nullptr

util/test/multiprocess_exec.h

Index: util/test/multiprocess_exec.h
diff --git a/util/test/multiprocess_exec.h b/util/test/multiprocess_exec.h
index a5c0f11c705034eeed7a343218bd0e1cf4014252..fecdfb37bbd16c22c3a643d8210efebbca195e51 100644
--- a/util/test/multiprocess_exec.h
+++ b/util/test/multiprocess_exec.h
@@ -46,7 +46,7 @@ class MultiprocessExec : public Multiprocess {
   //! \param[in] arguments The command-line arguments to pass to the child
   //!     process in its `argv[]` vector. This vector must begin at `argv[1]`,
   //!     as \a command is implicitly used as `argv[0]`. This argument may be
-  //!     `NULL` if no command-line arguments are to be passed.
+  //!     `nullptr` if no command-line arguments are to be passed.
   void SetChildCommand(const std::string& command,
                        const std::vector<std::string>* arguments);