Treat also undefined as null and false

[mifi]

in deserializeUser

prevent obscure issues like #6 (comment) #6 (comment)

because many query builders like knex return undefined if a model (e.g. user) is not found

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[mifi]

If this PR is too much of a breaking change, I alternatively created a PR to update the docs to make this more clear: passport/www.passportjs.org#109