Fix nil memoization of tokens

[gastronleroux]

The problem

Memoization of tokens doesn't work if those turn out to be missing. This leads to constant token lookup when accessing it or token-dependent resources like current_devise_api_token.

The solution

Instead of checking if the instance variable is truthy, check if it's defined.

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Great gem, by the way! 💯

[nejdetkadir]

thank u for your contribution 🙌🏻 @gastronleroux