broker rejects connect with empty will message

[dirkfeytons]

A client that has the will flag set, a will topic specified but an empty will message is rejected by the broker with the error message "will flag set, but will topic/message not present in payload".
It seems the code is confusing an empty will message (i.e. field present but with the length set to 0, which is valid) with the absence of the field (which is not according to the spec).

A change like below seems to fix it but I'm not 100% sure this is indeed the right fix:

diff --git a/amqtt/codecs.py b/amqtt/codecs.py
index 45cb7c5..1057fa6 100644
--- a/amqtt/codecs.py
+++ b/amqtt/codecs.py
@@ -83,7 +83,10 @@ async def decode_data_with_length(reader) -> bytes:
     """
     length_bytes = await read_or_raise(reader, 2)
     bytes_length = unpack("!H", length_bytes)
-    data = await read_or_raise(reader, bytes_length[0])
+    if bytes_length[0]:
+        data = await read_or_raise(reader, bytes_length[0])
+    else:
+        data = b""
     return data