Bug 17970: Fix GetPreparedLetter behavior if nothing to substitute

Bug 17970: Fix GetPreparedLetter behavior if nothing to substitute

From C4::Letters::GetPreparedLetter:

    my $tables = $params{tables};
    my $substitute = $params{substitute};

    $tables || $substitute || $repeat
       or carp( "ERROR: nothing to substitute - both 'tables' and 'substitute' are empty" ),
          return;

So if the parameter tables or substitute is passed but does not contain anything, it will not warn as expected.

Test plan:
1/ Apply the patch with tests
2/ Confirm that they do not pass
3/ Apply this patch
4/ Confirm that the tests now pass

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>