a11y atspi: add null checks in table iface methods

a11y atspi: add null checks in table iface methods

Origin: upstream, https://invent.kde.org/qt/qt/qtbase/-/commit/076da096464a5d3f
Last-Update: 2025-03-24
Bug: https://bugs.debian.org/1081682

Add null checks to cover the cases where QAccessibleTableInterface::cellAt
returns nullptr (which happens e.g. when called with invalid indices via
AT-SPI) or where the cell object doesn't implement the
QAccessibleTableCellInterface, which would previously result in crashes.

Cherry-picked into 5.15 as it fixes a crash in popular accessibility client
software. Conflict resolution: remove C++17'isms (`if` with initializer).

Gbp-Pq: Name a11y_null_checks.diff