avoid croak on latest CVS version of MARC::Record

avoid croak on latest CVS version of MARC::Record

The semantics of MARC::Record->delete_field() changed
slightly in the latest CVS version of MARC::Record.
delete_field() will now croak if passed an undef
scalar, while the version of MARC::Record from CPAN
accepts that input.  Since some packages of MARC::Record
in the wild appear to be based on the CVS version, not
the CPAN version, added a check where needed to avoid
the croak.

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