UserSetting VolumeDisplayOffset throws error when empty

[MathijsNL]

First of all thanks for this great tool. This seems to be the only working solution on the entire internet!

As in the title, the Offset in pixels can be empty and throws an error, just a minor thing though.

https://github.com/UnlimitedStack/HideVolumeOSD/blob/master/HideVolumeOSD/UserSettings.cs#L146

Settings.Default.VolumeDisplayOffset = int.Parse(textBoxOffset.Text);