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.

Lib/test/test_select.py

@@ -46,7 +46,7 @@ class SelectTestCase(unittest.TestCase):
         self.assertIsNot(r, x)
         self.assertIsNot(w, x)
 
-    @unittest.skipUnless(hasattr(os, 'popen'), "need os.popen()")
+    @support.requires_fork()
     def test_select(self):
         code = textwrap.dedent('''
             import time

Misc/NEWS.d/next/Library/2022-03-14-09-26-42.bpo-40280.2-k8TV.rst

@@ -0,0 +1 @@
+:func:`select.select` now passes ``NULL`` to ``select`` for each empty fdset.

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)