Interpolation with empty string literal ${''} evaluates to undefined

[ghost]

See #46

Example:

import styled from 'styled-components'

const Container = styled.div`
  ${''}
  ${''}
  max-width: 30rem;
  padding: 1rem;
`

export default Container

Second rule of the styles triggers Unexpected duplicate "undefined" [declaration-block-no-duplicate-properties] error.

[emilgoldsmith]

This is the same cause as #55 and should now be fixed.

edit: And the PR that should've fixed it is #59