PHP-1355: Fix collection/index enumeration for empty first batch

This also removes unnecessary scoping in mongo_db_list_collections_command().
php_mongocommandcursor_is_valid() should check started_iterating

This isn't necessary for internal code that calls this function, which sets started_iterating accordingly; however, it makes sense to have this check in the real function instead of the delegating PHP method.
PHP-1357: Fix log_response_header segfault for missing cursor query