No value no css

```jsx
const Line = styled(div)`
  background-color: ${props => props.bgColor};
  margin-left: ${props => props.margin};
`
```

if `bgColor='#fff'` and `margin=''`，
```css
{
    --b9s1f2v-1: #fff;
}

.b9s1f2v{
  background-color: var(--b9s1f2v-1);
}
```
Just like this, `margin` has no value, and the generated css has no `margin-left`. How to implement it.🤔

