Padding, Keepalive and Drop cells should have random payloads

tor-spec says:

   Link padding can be created by sending PADDING or VPADDING cells
   along the connection; relay cells of type "DROP" can be used for
   long-range padding.  The contents of a PADDING, VPADDING, or DROP
   cell SHOULD be chosen randomly, and MUST be ignored.

https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1534

But padding cells sent by channelpadding_send_padding_cell_for_callback() and keepalive cells sent by run_connection_housekeeping() have a payload of all zero bytes.

I don't know if this is a security issue or not. It is probably ok, unless Tor has compression enabled on its TLS connections. If compression is enabled, all the padding data size calculations will be wrong.