Modify require-return-type about annotateUndefined

[HsuTing]

With the following:

async () => {};

This code will throw error when the setting is ['always', { 'annotateUndefined': false }]. The async function is return Promise<void>, but I think this is the same as void for async function. So, this PR is used to make Promise<void> like void when using async function.

[HsuTing]

Hello, will this problem be solved?

[gajus]

Thank you

[gajus]

🎉 This PR is included in version 3.2.1 🎉

The release is available on:

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

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