Bug 15757: Make issuelength default to null instead of 0

When editing circ rules, if the Loan period "issuelength" is not defined
(empty string), the default value was 0, not it's inserted in the DB as
NULL.

Test plan:
1/ Create or edit a circ rule
2/ Do not fill the Loan period column
3/ Save
=> Without this patch, the value was 0
=> With this patch it's now an empty string (in the DB it's set to NULL)

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Works as described.
No errors

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

admin/smart-rules.pl

@@ -140,6 +140,7 @@ elsif ($op eq 'add') {
     $maxonsiteissueqty =~ s/\s//g;
     $maxonsiteissueqty = undef if $maxonsiteissueqty !~ /^\d+/;
     my $issuelength  = $input->param('issuelength');
+    $issuelength = $issuelength eq q{} ? undef : $issuelength;
     my $lengthunit  = $input->param('lengthunit');
     my $hardduedate = $input->param('hardduedate') || undef;
     $hardduedate = eval { dt_from_string( $input->param('hardduedate') ) } if ( $hardduedate );