[keyboard applet] fix TypeError garbage in log

While unharmful (JS checks array bounds), these errors pollute log every
time the applet starts:

file:///usr/share/plasma/plasmoids/org.kde.plasma.keyboardlayout/
contents/ui/main.qml:16: TypeError: Cannot read property 'displayName'
of undefined
file:///usr/share/plasma/plasmoids/org.kde.plasma.keyboardlayout/
contents/ui/main.qml:17: TypeError: Cannot read property 'longName' of
undefined
file:///usr/share/plasma/plasmoids/org.kde.plasma.keyboardlayout/
contents/ui/main.qml:18: TypeError: Cannot read property 'shortName' of
undefined

CCBUG: 432029