wireprotov2: don't emit empty frames

Staring at logs revealed the presence of empty frames that should have
contained payload. Let's stop that from happening.

Differential Revision: https://phab.mercurial-scm.org/D4925

mercurial/wireprotoframing.py

@@ -531,6 +531,9 b' class bufferingcommandresponseemitter(ob'
         # * If a chunk causes us to go over our buffering limit, we flush
         #   and then buffer the new chunk.
 
+        if not data:
+            return
+
         if len(data) > self._maxsize:
             for frame in self._flush():
                 yield frame
@@ -573,6 +576,9 b' class bufferingcommandresponseemitter(ob'
         self._chunks[:] = []
         self._chunkssize = 0
 
+        if not payload:
+            return
+
         yield self._stream.makeframe(
             self._requestid,
             typeid=FRAME_TYPE_COMMAND_RESPONSE,

tests/test-wireproto-serverreactor.py

@@ -394,7 +394,6 b' class ServerReactorTests(unittest.TestCa'
             b'1 2 0 command-response continuation Y\x80d',
             b'1 2 0 command-response continuation %s' % first,
             b'1 2 0 command-response continuation %s' % second,
-            b'1 2 0 command-response continuation ',
             b'1 2 0 command-response eos '
         ])