Bug 6521 - allow blank cardnumbers to not trigger "already in use"

Bug 6521 - allow blank cardnumbers to not trigger "already in use"

Now a blank cardnumber will get stored as a null in the database, which
is a value that allows duplicates. As such, if cardnumbers aren't
mandatory, then you can actually save a user with it being blank.

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit 785af7310daa4633359755add9474e81f108d022)

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