Fix undefined variable notice in print_manage_menu()

When the manage_proj_page.php is called for a user with minimal rights
(e.g. a reporter granted manager role on a single project), the
print_manage_menu() function issues a couple of messages (depending on
$g_display_errors settings):

SYSTEM NOTICE: Undefined variable: t_pages
SYSTEM WARNING: Invalid argument supplied for foreach()

This commit prevents these errors by initializing $t_pages to an empty
array.