[lookandfeel] Fix access to undefined needsPassword role with kscreenlocker

When running this:

~/kde/usr/lib/libexec/kscreenlocker_greet --testing --theme ~/kde/src/plasma-workspace/lookandfeel

it used to generate error like this:

/usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/components/UserList.qml:61:9: Unable to assign [undefined] to bool

Because not every users list model provides all the properties that delegate expected.