[Exim] Wishlist: Skip transport_filter if the expansion is …

I've got this transport_filter:

    transport_filter = "\"${extract{spamcmd}{$address_data} {$value}{/bin/cat}}\""

..which works fine, though I feel that it should have been a way of
simply skipping the filter, for example by using 'fail' instead of
'{/bin/cat}' .. after all it's fairly crackbrained to fork two(?) times
and shuffle data around when the message is anyway doomed to pass through
the filter unmodified. :-)

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Tore Anderson