minifier: should not remove undefined from return inside async generators

[sapphi-red]

async function* foo() { return undefined }

is compressed into

async function* foo() { return }

but this has a different semantics (see esbuild's changelog).

For example, this code behaves differently after minification.

async function* foo() { return undefined }
const p = foo().next().then(() => console.log('foo'))
Promise.resolve().then(() => {console.log('a')})

playground
(Before: a -> foo, After: foo -> a)

(This is not something I met with real world code)

[sapphi-red]

fixed by #8439