tools: ynl: move the new line in NlMsg __repr__

We add the new line even if message has no error or extack,
which leads to print(nl_msg) ending with two new lines.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>