Allow reason empty string to match None on closing

This was originally per #153 but I felt it didn't make sense to
change. However, it does and without this change the autobahn tests
fail.

Author: pgjones

src/wsproto/frame_protocol.py

@@ -571,7 +571,7 @@ def close(self, code: Optional[int] = None, reason: Optional[str] = None) -> byt
         payload = bytearray()
         if code is CloseReason.NO_STATUS_RCVD:
             code = None
-        if code is None and reason is not None:
+        if code is None and reason:
             raise TypeError("cannot specify a reason without a code")
         if code in LOCAL_ONLY_CLOSE_REASONS:
             code = CloseReason.NORMAL_CLOSURE