more error handling for AS2 objects without id or URL

follows b7e44c9295. fixes https://console.cloud.google.com/errors/CLnu14G46vHLOg

common.py

@@ -351,6 +351,8 @@ def redirect_wrap(url):
     https://github.com/snarfed/bridgy-fed/issues/16#issuecomment-424799599
     https://github.com/tootsuite/mastodon/pull/6219#issuecomment-429142747
     """
+    if not url:
+        return url
     if url.startswith(REDIRECT_PREFIX):
         return url
     return REDIRECT_PREFIX + url

tests/test_common.py

@@ -64,3 +64,7 @@ class CommonTest(testutil.TestCase):
         with self.assertRaises(exc.HTTPBadGateway):
             resp = common.get_as2('http://orig')
 
+    def test_redirect_wrap_empty(self):
+        self.assertIsNone(common.redirect_wrap(None))
+        self.assertEqual('', common.redirect_wrap(''))
+