Interpolations with a prefix/suffix that are undefined result in the prefix/suffix being added to the inline style

[itsdouges]

export const HorizontalStack = styled.div`
  margin-top: ${(props) => props.spacing}rem;
`;

Results in:

--_1g0t409: rem;

Instead it should just not be defined.

This could potentially be fixed in conjunction with #333

---

We can introduce another util that will handle this logic. Behaviour required:

1. Handle adding suffix/prefix only if the variable is defined
2. If the variable is undefined return an empty variable

function interpolation(variable?: string | number, suffix?: string, prefix?: string): string;