UploadPackHandler must return if objects_iter is empty

objects_iter is not None if client does not want any refs.
This patch avoids UploadPackHandler to continue pack creation and raise
GitProtocolError "Server attempts to access capability before
asking client".

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>

dulwich/server.py

@@ -277,7 +277,7 @@ class UploadPackHandler(Handler):
 
         # Did the process short-circuit (e.g. in a stateless RPC call)? Note
         # that the client still expects a 0-object pack in most cases.
-        if objects_iter is None:
+        if len(objects_iter) == 0:
             return
 
         self.progress("dul-daemon says what\n")