qmlcachegen generates invalid cpp when using ?.

Initially we found that a source file from kdeplasma-addons failed to build:

[   80s] /home/abuild/rpmbuild/BUILD/kdeplasma6-addons-6.3.80git.20250515T163246_36de84f9-build/kdeplasma-addons-6.3.80git.20250515T163246~36de84f9/build/applets/comic/.rcc/qmlcache/org.kde.plasma.comic_qml/ComicBottomInfo_qml.cpp:2343:27: error: ‘class QString’ has no member named ‘isUndefined’
[   80s]  2343 | if (r2_4.isNull() || r2_4.isUndefined())

The reason for that is that the referenced variable has (according to .qmltypes of the module) a type of QString, i.e. can't be null or undefined, so using the .? operator here is unnecessary, but not broken.

Reproducer:

import QtQuick

Text {
	id: root
	property string mystr: "asdf"

	// Warning: test.qml:10:10: Cannot convert from qlonglong to QJSPrimitiveValue [compiler]
	// text: (root.mystr?.length > 0) ? root.mystr : ""
	// Fails to build!
	text: (root.mystr?.length) ? root.mystr : ""
}

Initially the build failure only happened on 32-bit platforms. The reason for that is that on 64-bit platforms, qmlcachegen was unable to generate cpp for the 64-bit length value used by the comparison.