fix: Don't break when spreading falsy value

[gpittarelli]

The following is valid React:

const props = null;
<div {...props} styleName='a' />;

On master, after #243, this compiles to:

const props = null;
<div {...props} className={"foo__a" + (" " + (props.className || ""))} />;

However, this fails because it is trying to read the className property of null.

This PR fixes this (and updates the tests) by also compiling in a falsy-check around props:

const props = null;
<div {...props} className={"foo__a" + (" " + (props ? props.className || "" : ""))} />;

[gajus]

🎉 This PR is included in version 5.2.4 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[gpittarelli]

Thanks!