Handle unassigned template literal

[Raathigesh]

This will make #29 work without breaking the extension but it doesn't solve #29 completely. Looks like we don't handle nested template literals very well but we could fix that separately as an enhancement.

[paustint]

@Raathigesh - Can you confirm that the behavior I am seeing is what is expected? If so, then this LGTM.

Works (does not pull in properties, but also does not throw exception)

export const SearchCentering = styled.div`
  width: 100%;
  max-width: ${'1200px'};
`;

Does not work

export const SearchCentering = styled.div`
  width: 100%;
  ${mediaMin.xxl`
  margin: 0 auto;
  max-width: 1200px;
  `};
`;

[Raathigesh]

Yes, That's something I want to look at separately. Thank you for the review.