blueprint: return nil instead of empty user slice in GetUsers()

[pastalian]

Callers of GetUser() do not expect to receive an empty slice, which is causing unexpected condition handling.

images/pkg/distro/rhel/rhel9/options.go

Lines 106 to 107 in e734c0c

	if customizations.GetUsers() != nil {
	w := fmt.Sprintf("Please note that user customizations on %q image type are deprecated and will be removed in the near future\n", t.Name())

images/pkg/distro/rhel/rhel9/options.go

Lines 210 to 211 in e734c0c

	(customizations.GetUsers() != nil || customizations.GetGroups() != nil) {
	return warnings, fmt.Errorf("edge-installer installer.kickstart.contents are not supported in combination with users or groups")

[achilleas-k]

Thanks for this. LGTM!