net: ipconfig: Avoid spurious blank lines in boot log

When dumping the name and NTP servers advertised by DHCP, a blank line
is emitted if either of the lists is empty. This can lead to confusing
issues such as the blank line getting flagged as warning. This happens
because the blank line is the result of pr_cont("\n") and that may see
its level corrupted by some other driver concurrently writing to the
console.

Fix this by making sure that the terminating newline is only emitted
if at least one entry in the lists was printed before.

Reported-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20201110073757.1284594-1-thierry.reding@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>