Bug 5708 : Get Correct Discount when levying rentalcharge

Bug 5708 : Get Correct Discount when levying rentalcharge

Branch can be a parameter so it should be included in the rule retrieval
Need to select the most appropriate rule from those returned
(was defaulting to first returned!)
Try to make what's happening a bit less opaque
null fields are returned as undefined not 'NULL' by DBI

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit 88cb47339a8eab4ff4b95956207bf2d1dcdffeff)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>