fix crash in Web.owns_id

fixes https://console.cloud.google.com/errors/detail/CPOsl7usgr3VUQ;time=P30D?project=bridgy-federated

tests/test_web.py

@@ -2517,6 +2517,9 @@ class WebUtilTest(TestCase):
         self.user.key.delete()
         self.assertIsNone(Web.owns_id('user.com'))
 
+        # extra /, urlparse thinks domain is None
+        self.assertFalse(Web.owns_id('https:///github.com/puddly'))
+
     def test_owns_id_returns_None(self, *_):
         self.user.manual_opt_out = True
         self.user.put()

web.py

@@ -72,7 +72,7 @@ def is_valid_domain(domain):
 
     Valid means TLD is ok, not blacklisted, etc.
     """
-    if not re.match(DOMAIN_RE, domain):
+    if not domain or not re.match(DOMAIN_RE, domain):
         # logger.debug(f"{domain} doesn't look like a domain")
         return False