gather: get_packages_to_gather returns a tuple

It always needs to return a tuple, otherwise there would be crashes.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/phases/gather/__init__.py

@@ -532,7 +532,7 @@ def get_packages_to_gather(compose, arch=None, variant=None, include_arch=True,
         be included in a list of packages.
     """
     if compose.conf["gather_source"] == "module":
-        return []
+        return ([], [])
 
     arches = [arch] if arch else compose.get_arches()