gather: Stop requiring comps file in nodeps

When there are no groups, we shouldn't try to read comps file (because
it may very well not be there).

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

pungi/phases/gather/methods/method_nodeps.py

@@ -103,6 +103,10 @@ def expand_groups(compose, arch, groups):
 
     :returns: A set of tuples (pkg_name, arch)
     """
+    if not groups:
+        # No groups, nothing to do (this also covers case when there is no
+        # comps file.
+        return set()
     comps_file = compose.paths.work.comps(arch, create_dir=False)
     comps = CompsWrapper(comps_file)
     packages = set()