UBUNTU: [Packaging] insertchanges: avoid double newline

UBUNTU: [Packaging] insertchanges: avoid double newline

BugLink: https://bugs.launchpad.net/bugs/1903293
When some changes have been already added to the changelog, like when using
insert-ubuntu-changes, and there are no other changes, we end up with two
newlines right after the stanza header.

Add a $skip_newline variable that allows us to skip that extra newline when
there are no other changes.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>