[api] fix None reason throwing error

disco/api/client.py

@@ -26,7 +26,7 @@ def optional(**kwargs):
 
 
 def _reason_header(value):
-    return optional(**{'X-Audit-Log-Reason': quote(to_bytes(value))})
+    return optional(**{'X-Audit-Log-Reason': quote(to_bytes(value)) if value else None})
 
 
 class APIClient(LoggingClass):

tests/test_reason.py

@@ -9,3 +9,11 @@ class TestReason(TestCase):
 
         _, kwargs = api.http.calls[0]
         self.assertEqual(kwargs['headers']['X-Audit-Log-Reason'], 'yo%20%F0%9F%92%BF%20test')
+
+    def test_null_reason(self):
+        from tests.utils import TestAPIClient
+        api = TestAPIClient()
+        api.guilds_channels_modify(1, 2, 3, reason=None)
+
+        _, kwargs = api.http.calls[0]
+        self.assertFalse('X-Audit-Log-Reason' in kwargs['headers'])