No null checking when accountFetcher returns Promise

[dmitrij-borchuk]

Looks like I have found a bug. When accountFetcher returns Promise there is no null checking.

Code example.

tokenize.validate(token, async (id, prefix) => {
  return null
})

This code raises an exception TypeError: Cannot convert undefined or null to object.

Here (https://github.com/cyyynthia/tokenize/blob/mistress/index.js#L117) we have checked for a null but in the case of Promise, this checking will pass value further. Later (https://github.com/cyyynthia/tokenize/blob/mistress/index.js#L120) it will pass the null to _finishValidation where the error raises.