Corrupted class names including string "null" generated from dynamic style with media query conditions

### Describe the issue

When using `stylex.create()` with dynamic style functions having a media query condition evaluating to `null`, StyleX generates class names that incorrectly include the raw string "null" in them leading to corrputed name, like `"nullxafpxz5"`. This corrupted class name results respective style getting completely ignored. According to the StyleX documentation, null values should be ignored and not included in the generated class names.

### Expected behavior

Conditional media query styles that return `null` should be ignored.

### Steps to reproduce

Following component should have `background-color: tomato` when viewport is wider than 600px:

```jsx
import * as stylex from "@stylexjs/stylex";

const styles = stylex.create({
highlightAt: (small, large) => ({
backgroundColor: {
default: null,
"@media (max-width: 599px)": small ? "red" : null,
"@media (min-width: 600px)": large ? "red" : null,
},
}),
});

const MyComponent = () => (

- **StyleX version**: 0.18.3
- **Plugins**: @stylexjs/babel-plugin, @stylexjs/postcss-plugin
- **Environments tested**: Astro.js dev server and production builds, Storybook

### Test case

Reproduction steps work as deterministic test case.

### Additional comments

Behavior is likely due to transpilation issue that can be reduced to very minimal Node.js program when standard stylex dependencies (@babel/core @babel/preset-react @stylexjs/babel-plugin) are in place. Self-contained reproduction repository: https://github.com/joonaojapalo/stylex-dynamic-mediaquery-repro

```js
// repro.js
const babel = require("@babel/core");

const input = `
import * as stylex from '@stylexjs/stylex';

const styles = stylex.create({
highlightAt: (small, large) => ({
backgroundColor: {
"@media (max-width: 599px)": small ? "red" : null,
"@media (min-width: 600px)": large ? "red" : null,
},
}),
});