[unity] SpineSkin Attribute does not include `None` on `includeNone=true`

[HaraldCsaszar]

Reported on the forum:
https://esotericsoftware.com/forum/d/25871-includenone--true-doesnt-return-none-in-dropdown-list

Currently <None> is not shown regardless of includeNone=true.
This is relevant when SpineSkin() parameter string startsWith filters every skin out, then only an empty list is displayed where nothing can be selected. A <none> entry shall be provided for this case, with old behaviour of hiding <none> becoming the default, thus includeNone shall now default to false.