Ensure DEFAULT is taken into account for matchVariant

[RobinMalfait]

This means that if you define your matchVariant as:

matchVariant('foo', (value) => '.foo-${value} &')

Then you can't use foo:underline, if you want to be able to use
foo:underline then you have to define a DEFAULT value:

matchVariant('foo', (value) => '.foo-${value} &', {
  values: {
    DEFAULT: 'bar'
  }
})

Now foo:underline will generate .foo-bar & as a selector!