stylix: wallpaper should be optional

[tbaumann]

I don't use the generative colour scheme, I mostly use stylix for the superb system wide theming together with a pre-defined colour scheme.

I find stylix.image should be optional when stylix.base16Scheme is used.

Arguably it should perhaps even be entirely optional. Instead of throwing a rather obscure error it should perhaps just not apply any theme or use a default one...

error: A definition for option `stylix.image' is not of type `path or package convertible to it'.

Or produce an error message specifying that stylix.image is not optional.

[danth]

We should probably support having null as the wallpaper and in that case simply disable anything which uses it.