SDKtool: Only write values into kit that were set on the commandline

Do not put empty values for settings into the kits: These will prevent
data from being editable in Qt Creator.

Task-number: QTCREATORBUG-12017
Change-Id: I10f67ed0038ba7e1a79cbf7b5658247a2d18b8eb
Reviewed-by: Eike Ziller <eike.ziller@digia.com>