Fix crash on serializing default-constructed QTimeZone
The serialization code neglected to check against null. Sinze zones are saved either by IANA ID or in our special OffsetFromUtc format, representing an invalid zone by a string that cannot possibly be a valid IANA ID will do.
Fixes: QTBUG-86019 Pick-to: 5.15 Pick-to: 5.12 Change-Id: I6882026403d00f8b254aab34c645f1cf8f9fcc2d Reviewed-by: Taylor Braun-Jones taylor@braun-jones.org Reviewed-by: Thiago Macieira thiago.macieira@intel.com (cherry picked from commit 14f3f419)