Add test to explicit check the absence regression in subprocess (issue #15592).

Patch by Chris Jerdonek.

Lib/test/test_subprocess.py

@@ -548,6 +548,18 @@ class ProcessTestCase(BaseTestCase):
         (stdout, stderr) = p.communicate("line1\nline3\n")
         self.assertEqual(p.returncode, 0)
 
+    def test_universal_newlines_communicate_input_none(self):
+        # Test communicate(input=None) with universal newlines.
+        #
+        # We set stdout to PIPE because, as of this writing, a different
+        # code path is tested when the number of pipes is zero or one.
+        p = subprocess.Popen([sys.executable, "-c", "pass"],
+                             stdin=subprocess.PIPE,
+                             stdout=subprocess.PIPE,
+                             universal_newlines=True)
+        p.communicate()
+        self.assertEqual(p.returncode, 0)
+
     def test_no_leaking(self):
         # Make sure we leak no resources
         if not mswindows: