registry: Fix regression preventing default None value for Regexp.

src/registry.py

@@ -768,17 +768,23 @@ class Regexp(Value):
                 # value of there parent change.
                 v = (s, utils.str.perlReToPythonRe(s))
             else:
-                v = ('', '')
+                v = None
         except ValueError as e:
             self.error(e)
         else:
             super().set(v)
 
     def __call__(self):
-        return self.value[1]
+        if self.value is None:
+            return None
+        else:
+            return self.value[1]
 
     def __str__(self):
-        return self.value[0]
+        if self.value is None:
+            return ''
+        else:
+            return self.value[0]
 
 class SeparatedListOf(Value):
     __slots__ = ()

test/test_registry.py

@@ -182,8 +182,6 @@ class ValuesTestCase(SupyTestCase):
         self.assertTrue(v().match('foo'))
         v.set('')
         self.assertEqual(v(), None)
-        self.assertRaises(registry.InvalidRegistryValue,
-                          v.setValue, re.compile(r'foo'))
 
     def testBackslashesKeys(self):
         conf.supybot.reply.whenAddressedBy.strings.get(':foo').set('=/*')