[ChangeLog][QtCore][QJsonDocument] Fixed a bug that caused
QJsonDocument's equality operator to crash if one of the operands was
default-constructed and the other wasn't.
Pick-to: 5.15
Fixes: QTBUG-85969
Change-Id: I5e00996d7f4b4a10bc98fffd1629f835f570ef6b
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>