GitHub source - Do not error on empty repository (#125)

The Catalog Importer's GitHub "Source" is able to read from all
repositories of an organisation, searching each repo's tree for files
matching specified file path patterns.

If a repository contains no matching files, no error is thrown. If a
repository is empty, it makes sense for the same optimistic behaviour to
be followed.

Though GitHub's `Response` type embeds the HTTP Response, we can't rely
on the 409 alone (The [API
docs](https://docs.github.com/en/rest/git/trees?apiVersion=2022-11-28)
describe 409 broadly as "conflict"), so I've opted to string match

Issue #68

source/source_github.go

@@ -139,6 +139,11 @@ func (s SourceGitHub) Load(ctx context.Context, logger kitlog.Logger) ([]*Source
 					"owner", target.Owner, "repo", target.Repo, "ref", target.Ref)
 				tree, _, err := client.Git.GetTree(ctx, target.Owner, target.Repo, target.Ref, true)
 				if err != nil {
+					if repositoryEmpty(err) {
+						logger.Log("msg", "GitHub repository is empty, skipping",
+							"owner", target.Owner, "repo", target.Repo, "ref", target.Ref)
+						return nil
+					}
 					return errors.Wrap(err, fmt.Sprintf("getting tree for '%s/%s' at ref %s", target.Owner, target.Repo, target.Ref))
 				}
 
@@ -222,3 +227,10 @@ func (s SourceGitHub) Load(ctx context.Context, logger kitlog.Logger) ([]*Source
 
 	return entries, nil
 }
+
+func repositoryEmpty(err error) bool {
+	if err == nil {
+		return false
+	}
+	return strings.Contains(err.Error(), "409 Git Repository is empty")
+}