If $limit is empty(), no limit is set, so we can
set the LIMIT statement to "" to achieve the same.
But an empty() $limit can be "0", so the
generated SQL statement could end with "0"
instead of the LIMIT statement. We catch this
with forcing an empty() $limit to "".
Fixes#636.
(No matter that this shouldn't even happen.)
Signed-off-by: Thomas Hochstein <thh@inter.net>