Do not show current day if registration date is unknown

The registration date field on the account details page currently
defaults to the current day if the user's registration date is unknown.
To avoid confusion, show "unknown" in these cases instead.

Fixes FS#51405.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

web/template/account_details.php

@@ -61,7 +61,11 @@
 				</tr>
 				<tr>
 					<th><?= __("Registration date:") ?></th>
+					<?php if ($row["RegistrationTS"]): ?>
 					<td><?= (new DateTime($row["RegistrationTS"]))->format('Y-m-d') ?></td>
+					<?php else: ?>
+					<td><?= __("unknown") ?></td>
+					<?php endif; ?>
 				</tr>
 				<?php if (has_credential(CRED_ACCOUNT_LAST_LOGIN)): ?>
 				<tr>