Replaced all occurances of tbl_keys[0]

If a table object has no primary key(s) its property _tbl_keys remains an empty array. Because of this trying to get the key as with the current implementation `_tbl_keys[0]` causes the following error to be shown:

Notice: Undefined offset: 0 in /srv/http/k2s.ubuntu.de/libraries/joomla/table/table.php on line 785

The less stringent way via `current([tbl_keys])` prevents this message to be thrown.