Bug 20076: (RM follow-up) Fix system preference boolean logic

Bug 20076: (RM follow-up) Fix system preference boolean logic

A Yes/No system preference must use 1 for Yes and 0 for No.
So "Send" for 1/Yes and "Don't send" for 0/No.
We add too much problems with double-negation boolean system preferences (such as dontmerge).

Previous patch changed default value to 1 in atomicupdate, do the same
in installer/data/mysql/mandatory/sysprefs.sql

Also to be consistant, sets options = NULL instead of '' in atomicupdate

Also removed useless added empty line in /misc/cronjobs/overdue_notices.pl

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>