Only collect arch_packages if actually generating any output.
Fixes: commit f09b9319 ("build,json: store arch_packages in profiles.json"(
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 3b0f698760ae3a62173a28f18e9e1e3adef9c492)