bugfix: handle subfield $0 in MARC for an item

bugfix: handle subfield $0 in MARC for an item

Note: C4::Biblio::GetMarcFromKohaField returns (0, 0) if
the kohafield is not defined in the MARC framework, but
in may places the return value is not checked correctly
and does not recognize a subfield $0 (zero).  It
would be better if GetMarcFromKohaField returned (undef, undef)
in that circumstance, but because of the number of places
where that function is used, the changed is deferred for
post 3.0.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>