Only call frame_split_size when there are frames (#3996)

This just becomes a no-op if an empty list is given (returning an empty
list). However there is no need to do this work if we don't need it. So
simply restrict calling `frame_split_size` to the case where frames are
present and compression will occur.

distributed/protocol/serialize.py

@@ -475,8 +475,8 @@ def serialize_bytelist(x, **kwargs):
     header, frames = serialize(x, **kwargs)
     if "lengths" not in header:
         header["lengths"] = tuple(map(nbytes, frames))
-    frames = sum(map(frame_split_size, frames), [])
     if frames:
+        frames = sum(map(frame_split_size, frames), [])
         compression, frames = zip(*map(maybe_compress, frames))
     else:
         compression = []