Bug 24361: Fix warning in C4::Record line 435

Bug 24361: Fix warning in C4::Record line 435

Use of uninitialized value in concatenation (.) or string at /kohadevbox/koha/C4/Record.pm line 435

Caught by a failing test:
    #   Failed test 'Export csv with wrong marcxml should raise a warning'
    #   at t/db_dependent/Exporter/Record.t line 113.
    # found warning:  problem with :721 : :1: parser error : Start tag expected, '<' not found
    # found warning: Use of uninitialized value in concatenation (.) or string at /kohadevbox/koha/C4/Record.pm line 435.
    # expected to find warning: (?^u:.*Start tag expected.*)

Test plan:
Make sure the tests pass now

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>