Bug 15757: Make issuelength default to null instead of 0

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>
(cherry picked from commit d46a664ef6b79f6ceee2c00629f5933b4eaedfc3)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>