dillon1000/react

[hir] Strip JSXEmptyExpression syntax

`JSXEmptyExpression` is never added to a React element's children [in 
`react.buildChildren`](https://github.com/babel/babel/blob/main/packages/babel-types/src/builders/react/buildChildren.ts), 
which is [used 
by](https://github.com/babel/babel/blob/main/packages/babel-plugin-transform-react-jsx/src/create-plugin.ts#L649) 
`plugin-transform-react-jsx`]. 

An alternative would be to represent JSX expressions differently in HIR, then 
codegen `JSXEmptyExpression`s back when we encounter an `EmptyExpression` 

```js 

-   children: Array<Place>, 

-   children: Array<Place | "EmptyExpression">, 

``` 

(We could also retain `JSXEmptyExpression` as an `InstructionValue` that 
produces a Primitive. However, this would make babel types in Codegen a bit more 
messy, as `JSXEmptyExpression` does not extend `Expression` (which currently is 
the result of every `InstructionValue`).)