Fix bug where getDetails() could invalidate icon url.

QQmlProperty map cannot remove values, only "clear" them. When they were
converted to a QVariantMap (for retrieving url) cleared values would
be sent as invalid QVariants, which caused the url fetch to fail.

Solution is to check for invalid qvariants.

Change-Id: Ifd9407b3217edbd2d5a1702915f82c46f3c5f498
Reviewed-by: Albin Olsson <albin.olsson@cybercom.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>