openpgp: TPKBuilder::autocrypt should not default to an empty UID

  - TPKBuilder::autocrypt created a TPK with a single User ID, as
    required by the Autocrypt specification.  Since no User ID was
    passed, it used the empty string.

  - An empty User ID is a bit surprising, and it is unclear if it is
    even a reasonable default (GnuPG rejects it).  But, even if the
    programmer is aware of this, adding a new user ID does not replace
    the empty User ID, and removing the empty User ID is a pain.

  - Change the API to better match typical usage: have the constructor
    take the User ID.

  - Nevertheless, preserve the flexibility by making the User ID
    optional to allow the caller to add a User ID later.  In this
    case, a non-autocrypt compliant TPK with no User ID is created
    instead of an empty User ID.

  - Closes #146.

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