Treat variants without comps groups as having all of them

If the `<groups></groups>` section is not specified in the variants XML
file, all groups will be used in this variant. The section must be
omitted completely, not just empty. This is (and was) correct according
to the DTD, it just lead to crash before.

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

pungi/phases/gather/sources/source_comps.py

@@ -48,8 +48,9 @@ class GatherSourceComps(pungi.phases.gather.source.GatherSourceBase):
         groups = set()
         comps = CompsWrapper(self.compose.paths.work.comps(arch=arch))
 
-        if variant is not None:
-            # get packages for a particular variant
+        if variant is not None and variant.groups:
+            # Get packages for a particular variant. If the variant has no
+            # groups listed, all groups will be used.
             comps.filter_groups(variant.groups)
 
         for i in comps.get_comps_groups():

tests/data/dummy-variants.xml

@@ -27,6 +27,13 @@
     </groups>
   </variant>
 
+  <variant id="Everything" name="Everything" type="variant">
+    <arches>
+      <arch>i386</arch>
+      <arch>x86_64</arch>
+    </arches>
+  </variant>
+
   <variant id="Client" name="Client" type="variant" has_optional="true">
     <arches>
       <arch>i386</arch>