bpo-40280: select: Use NULL for empty fdset (GH-31865)

wasm32-emscripten does not support exceptfds and requires NULL. Python
now passes NULL instead of a fdset pointer when the input list is empty.
This works fine on all platforms and might even be a tiny bit faster.

Modules/selectmodule.c

@@ -330,7 +330,12 @@ select_select_impl(PyObject *module, PyObject *rlist, PyObject *wlist,
     do {
         Py_BEGIN_ALLOW_THREADS
         errno = 0;
-        n = select(max, &ifdset, &ofdset, &efdset, tvp);
+        n = select(
+            max,
+            imax ? &ifdset : NULL,
+            omax ? &ofdset : NULL,
+            emax ? &efdset : NULL,
+            tvp);
         Py_END_ALLOW_THREADS
 
         if (errno != EINTR)