T2345: Replace instances of NULL with nullptr

Per guidance from Axel, revert lib/kokkos and lib/colvars
to remove nullptr changes.

lib/kokkos/cmake/compile_tests/pthread.cpp

@@ -4,11 +4,11 @@ void* kokkos_test(void* args) { return args; }
 
 int main(void) {
   pthread_t thread;
-  /* Use nullptr to avoid C++11. Some compilers
+  /* Use NULL to avoid C++11. Some compilers
      do not have C++11 by default.  Forcing C++11
      in the compile tests can be done, but is unnecessary
   */
-  pthread_create(&thread, nullptr, kokkos_test, nullptr);
-  pthread_join(thread, nullptr);
+  pthread_create(&thread, NULL, kokkos_test, NULL);
+  pthread_join(thread, NULL);
   return 0;
 }