When setting optional parameters from the GUI they can not be unset anymore

[matthias-mayr]

If a skill with optional parameters is started with the optional parameters being set, this parameter can not be "unset" or not set in subsequent executions.
So even if one selects the empty entry for that optional parameter in the SkiROS GUI, the next time the skill is started, it would still receive the old value.
Setting new values works as expected, but not setting a value is currently not possible.

This can be cured by restarting the GUI.