[ProcessModel] Return invalid QVariant() for when window is not known

Authored by broulik on Apr 9 2018, 9:56 AM.

Description

[ProcessModel] Return invalid QVariant() for when window is not known

This communicates "unknown" better than a zero. The KSysGuardProcessList also seems to expect this.

Differential Revision: https://phabricator.kde.org/D12000

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