[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`).)

compiler/forget/src/HIR/BuildHIR.ts

@@ -1352,9 +1352,10 @@ function lowerExpression(
       const expr = exprPath as NodePath<t.JSXElement>;
       const opening = expr.get("openingElement");
       const tag = lowerJsxElementName(builder, opening.get("name"));
-      const children = expr
+      const children: Array<Place> = expr
         .get("children")
-        .map((child) => lowerJsxElement(builder, child));
+        .map((child) => lowerJsxElement(builder, child))
+        .filter(notNull);
       const props: Array<JsxAttribute> = [];
       for (const attribute of opening.get("attributes")) {
         if (attribute.isJSXSpreadAttribute()) {
@@ -1426,9 +1427,10 @@ function lowerExpression(
     }
     case "JSXFragment": {
       const expr = exprPath as NodePath<t.JSXFragment>;
-      const children = expr
+      const children: Array<Place> = expr
         .get("children")
-        .map((child) => lowerJsxElement(builder, child));
+        .map((child) => lowerJsxElement(builder, child))
+        .filter(notNull);
       return {
         kind: "JsxFragment",
         children,
@@ -1950,27 +1952,22 @@ function lowerJsxElement(
     | t.JSXElement
     | t.JSXFragment
   >
-): Place {
+): Place | null {
   const exprNode = exprPath.node;
   const exprLoc = exprNode.loc ?? GeneratedSource;
   if (exprPath.isJSXElement() || exprPath.isJSXFragment()) {
     return lowerExpressionToTemporary(builder, exprPath);
   } else if (exprPath.isJSXExpressionContainer()) {
     const expression = exprPath.get("expression");
-    if (!expression.isExpression()) {
-      builder.errors.push({
-        reason: `(BuildHIR::lowerJsxElement) Handle ${expression.type} expressions`,
-        severity: ErrorSeverity.Todo,
-        nodePath: expression,
-      });
-      const place = lowerValueToTemporary(builder, {
-        kind: "UnsupportedNode",
-        node: exprNode,
-        loc: exprLoc,
-      });
-      return { ...place };
+    if (expression.isJSXEmptyExpression()) {
+      return null;
+    } else {
+      invariant(
+        expression.isExpression(),
+        `(BuildHIR::lowerJsxElement) Expected Expression but found ${expression.type}!`
+      );
+      return lowerExpressionToTemporary(builder, expression);
     }
-    return lowerExpressionToTemporary(builder, expression);
   } else if (exprPath.isJSXText()) {
     const place = lowerValueToTemporary(builder, {
       kind: "JSXText",
@@ -2465,3 +2462,7 @@ function gatherCapturedDeps(
 
   return { identifiers: [...capturedIds], refs: [...capturedRefs] };
 }
+
+function notNull<T>(value: T | null): value is T {
+  return value !== null;
+}

compiler/forget/src/__tests__/fixtures/compiler/error.todo-kitchensink.expect.md

@@ -120,15 +120,6 @@ let moduleLocal = false;
   14 |
   15 |   <Button haha={1}></Button>;
 
-[ReactForget] TodoError: (BuildHIR::lowerJsxElement) Handle JSXEmptyExpression expressions
-  14 |
-  15 |   <Button haha={1}></Button>;
-> 16 |   <Button>{/** empty */}</Button>;
-     |            ^^^^^^^^^^^^
-  17 |
-  18 |   const j = function bar([quz, qux], ...args) {};
-  19 |
-
 [ReactForget] TodoError: (BuildHIR::lower) Handle RestElement params
   16 |   <Button>{/** empty */}</Button>;
   17 |

compiler/forget/src/__tests__/fixtures/compiler/jsx-empty-expression.expect.md

@@ -0,0 +1,34 @@
+
+## Input
+
+```javascript
+export function Component(props) {
+  return (
+    <div>
+      {}
+      {props.a}
+    </div>
+  );
+}
+
+```
+
+## Code
+
+```javascript
+export function Component(props) {
+  const $ = React.unstable_useMemoCache(2);
+  const c_0 = $[0] !== props.a;
+  let t0;
+  if (c_0) {
+    t0 = <div>{props.a}</div>;
+    $[0] = props.a;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/compiler/jsx-empty-expression.js

@@ -0,0 +1,8 @@
+export function Component(props) {
+  return (
+    <div>
+      {}
+      {props.a}
+    </div>
+  );
+}