Backport Revert "don't allow GeneralNames to be an empty list (#4128)" (#4161) (#4163)

* Revert "don't allow GeneralNames to be an empty list (#4128)" (#4161)

This breaks the urllib3 tests, as well as several in-the-wild certs

This reverts commit 388d1bd3e9.

* changelog

* Don't reference undocumented class

CHANGELOG.rst

@@ -1,6 +1,14 @@
 Changelog
 =========
 
+.. _v2-2-1:
+
+2.2.1 - 2018-03-20
+~~~~~~~~~~~~~~~~~~
+
+* Reverted a change to ``GeneralNames`` which prohibited having zero elements,
+  due to breakages.
+
 .. _v2-2:
 
 2.2 - 2018-03-19

src/cryptography/x509/extensions.py

@@ -1160,9 +1160,6 @@ class Extension(object):
 class GeneralNames(object):
     def __init__(self, general_names):
         general_names = list(general_names)
-        if len(general_names) == 0:
-            raise ValueError("Must supply at least one general name")
-
         if not all(isinstance(x, GeneralName) for x in general_names):
             raise TypeError(
                 "Every item in the general_names list must be an "

tests/x509/test_x509_ext.py

@@ -2052,10 +2052,6 @@ class TestGeneralNames(object):
                 [x509.DNSName(u"cryptography.io"), "invalid"]
             )
 
-    def test_does_not_allow_empty_list(self):
-        with pytest.raises(ValueError):
-            x509.GeneralNames([])
-
     def test_repr(self):
         gns = x509.GeneralNames(
             [