Fix: prevent crash and warnings on items export for Problems and Changes

[Rom1-B]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !43955
• Items linked to a Problem or Change may not have serial/otherserial columns (e.g. Software, User, Budget). Accessing these missing array keys caused a fatal TypeError in item_problem (crashing PDF generation) and a PHP warning flood in change_item. Use null-coalescing to default to an empty string.

Also to be done in 11