quick: Fix Merkuro validBackend undefined

Florian RICHERrequested to merge
work/florianricher/remove-usage-of-valid-backend into master

When i try to update the identity. I got this error on Merkuro:

qrc:/qt/qml/org/kde/kidentitymanagement/IdentityEditorPage.qml:70:9: Unable to assign [undefined] to bool

I appear in Merkuro backend we don't have validBackend property.

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