Verify QVariant is valid before casting to Client*

If the ClientModel does not contain any Clients, which can
happen if there is no desktop window, accessing the data of a
ModelIndex returns an invalid QVariant. Because of that it
needs to be ensured that the QVariant is valid before trying to
cast it to a Client Pointer.

BUG: 304620
FIXED-IN: 4.9.1
REVIEW: 105935