fix(cc_apt_configure): avoid unneeded call to apt-install

[aciba90]

Proposed Commit Message

If no apt-sources config is given, then no apt dependencies are required to be installed.
This avoids unneeded calls to apt-update and apt-install.

The problem was introduced in 015543d304.

Additional Context

#4441

Test Steps

lxc launch ubuntu-daily:devel mm
lxc shell mm

# without this patch the following happens while cc_apt_configure runs, and no package is installed:
$ grep -E 'apt-update|apt-install' /var/log/cloud-init.log
2023-10-17 14:50:41,775 - util.py[DEBUG]: apt-update [eatmydata apt-get --option=Dpkg::Options::=--force-confold --option=Dpkg::options::=--force-unsafe-io --assume-yes --quiet update] took 8.957 seconds
2023-10-17 14:50:42,142 - util.py[DEBUG]: apt-install [eatmydata apt-get --option=Dpkg::Options::=--force-confold --option=Dpkg::options::=--force-unsafe-io --assume-yes --quiet install] took 0.366 seconds

# with this patch no output expected

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[blackboxsw]

Fix is good. I've added supplemental unittests for various configs which can be provided to _ensure_dependencies. When this passes we can merge this and reflect to release branches