Fix undefined variable issue

[VoidChen]

Fixes issue #33

[aleho]

Thanks for your input!

This should rather be done be using an empty string. The way they're disabled, if you take a look in dconf-editor e.g, shows something like "[ '' ]" as value for an array setting. I missed that when setting the default for value to undefined.

[VoidChen]

Thanks for your reply. I try the latest version, it works!