Don't die on cascaded updates to deleted rows.

DBIx::Class cascades updates by default on has_one and might_have relationships. If in the same transaction the related row got deleted and would be affected by a cascaded update, AuditLog dies with an DBIx::Class::Schema::AuditLog::txn_do(): Can't call method "get_columns" on an undefined value at /usr/lib/perl5/site_perl/5.16.0/DBIx/Class/AuditLog.pm line 31. This happens because AuditLog tries to fetch the row freshly from the database and tries to access its data. Commit 4f09d5781c8e30b57436f567f07b6024e8c227ab fixes this.