QXmlDefaultHandler: fix uninit'ed d member

Essentially the same change as the recent one for QXmlAttributes, so
see there for details, with the following important difference: There
is nothing here that actually reads the member, not even SMFs. So no
UB here, this is just cleanup.

So just leave it uninit'ed and hope that Coverity understands the
[[maybe_unused]] added in macro form so it stops complaining. If it
doesn't, we will add = nullptr, like for QXmlAttributes.

Pick-to: 6.12 6.11 6.8
Coverity-Id: 11423
Change-Id: Idfc2182b63ce5e78eaa68c5e778bcaa9dfc47c24
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>