Firebase: Adjust undefined import causing IDE annoyances

[camsjams]

This change attempts to prevent the auto-importing of an exported undefined value from firebase/empty-import, which predominantly occurs in VS Code.

Fixes #2203

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[Feiyang1]

Thanks so much!
It probably doesn't make a huge difference, but I'd prefer to change the typing to

declare namespace empty {

}
export = empty;

Can you please also sign the CLA, so we can approve and merge the change?

[camsjams]

@googlebot I signed it!

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CLAs look good, thanks!

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[camsjams]

@Feiyang1
Regarding the CLA, I forgot to reply to the bot.

I agree on the typing, I changed it to an empty namespace per your comment.

[Feiyang1]

Looks good, Thanks!

[danielkcz]

May I pretty please ask to get this published soon? It doesn't solve only the reported issue but for me, it's crashing tsserver and making Typescript experience very unpleasant.

microsoft/TypeScript#35036 (comment)