Add test to make sure there is a variants table

[rm-code]

For example love.window.restore caused some issues for love-atom, because the empty variants table was omitted.

        {
            name = 'restore',
            description = 'Restores the size and position of the window if it was minimized or maximized.',
            variants = {
                {}
            }
        },

@positive07 Maybe you have some time to take a look at this? :)

[pablomayobre]

Would it be a good idea to just do #45 and make a simple test thingy that loops over what needs to be there?