Allow importing modules without a Koji tag

These module builds will basically act as metadata-only module builds.
This will be more useful as additional features stem from these types
of builds.

module_build_service/resolver/DBResolver.py

@@ -245,6 +245,11 @@ class DBResolver(GenericResolver):
                 if not build:
                     raise RuntimeError(
                         'Buildrequired module %s %r does not exist in MBS db' % (br_name, details))
+
+                # If the buildrequire is a meta-data only module with no Koji tag set, then just
+                # skip it
+                if build.koji_tag is None:
+                    continue
                 module_tags.setdefault(build.koji_tag, [])
                 module_tags[build.koji_tag].append(build.mmd())