Bug 6475 - Edit it's not possible in subfield "0" in MARC framework

Bug 6475 - Edit it's not possible in subfield "0" in MARC framework

When trying to modify the subfield "0" on a field in the
MARC subfield structure admin -> Tag xxx Subfield Constraints,
the boolean condition on liblibrarian fails for the initial "0" value.
Modified condition to allow the value "0"

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit 042f89e939984b2b9e568622e0b99c95db09750e)

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