Check issue labels slice length before calling xorm Insert(#20655) (#20836)

Backport #20655

Fix #20654

Co-authored-by: Gabriel Vasile <gabriel.vasile@email.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

services/repository/template.go

@@ -21,6 +21,11 @@ func GenerateIssueLabels(ctx context.Context, templateRepo, generateRepo *repo_m
 	if err != nil {
 		return err
 	}
+	// Prevent insert being called with an empty slice which would result in
+	// err "no element on slice when insert".
+	if len(templateLabels) == 0 {
+		return nil
+	}
 
 	newLabels := make([]*issues_model.Label, 0, len(templateLabels))
 	for _, templateLabel := range templateLabels {