Both POSIX, the Linux documentation and UNIX Network Programming volume 1
by Stephens state that NULL is accepted.

This avoid the following compiler message:

../../examples/player_example.c: In function ‘main’:
../../examples/player_example.c:839:22: warning: passing argument 2 to ‘restrict’-qualified parameter aliases with argument 4 [-Werror=restrict]
  839 |           n=select(n,&empty,&writefs,&empty,&timeout);
      |                      ^~~~~~          ~~~~~~
../../examples/player_example.c:843:18: warning: passing argument 2 to ‘restrict’-qualified parameter aliases with argument 4 [-Werror=restrict]
  843 |         select(n,&empty,&writefs,&empty,NULL);
      |                  ^~~~~~          ~~~~~~