jsonrpc: Don't assert for 0 remotes in jsonrpc_session_open_multiple().

jsonrpc: Don't assert for 0 remotes in jsonrpc_session_open_multiple().

It's pretty easy to get 0 remotes here from ovn-northd if you specify
--ovnnb-db='' or --ovnnb-db='   ' on the command line.  The internals
of jsonrpc_session aren't equipped to cope with that, so just add a
dummy remote instead.

Acked-by: Numan Siddique <numans@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>