dlm: fix lowcomms_connect_node for sctp

The recently added dlm_lowcomms_connect_node() from
391fbdc5

 does not work
when using SCTP instead of TCP.  The sctp connection code
has nothing to do without data to send.  Check for no data
in the sctp connection code and do nothing instead of
triggering a BUG.  Also have connect_node() do nothing
when the protocol is sctp.

Signed-off-by: David Teigland <teigland@redhat.com>