Consider a case when users.inbox is nil (Fix 3241)

Fix "Followers only" does not send a note to other instances. (#3241)

Closes #3241 (closed)

users.inbox can be nil:

pleroma=# \d users
                                                                                   Table "public.users"
                Column                |              Type              | Collation | Nullable |                                          Default
--------------------------------------+--------------------------------+-----------+----------+-------------------------------------------------------------------------------------------
...
 inbox                                | text                           |           |          |

But currently, it is not handled. In this MR, handle cases when inbox is nil.

Checklist

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