bugfix: adding children to emotion jsx function

[marceloadsj]

Summary

A bug on the xstyled/emotion jsx API makes the children of tags with "css" props to not be rendered:

<div css={{...}}>
  <p>A paragraph</p> // currently, this will not be rendered
</div>

So, this PR fixes this issue and creates some tests to ensure its stability.

Test plan

3 tests were created:

• Empty
  does not render any content
• with a single children renders the content
• with multiple children renders all elements

Before:

After: