Fix answering questions

First, thanks for updating this one! It's really great!
I'm not sure this is base problem, but I got this error when answering the questions.

```
  Rendered mailer/_issue.text.erb (3.8ms)
  Rendered plugins/question_plugin/app/views/question_mailer/answered_question.text.erb within layouts/mailer (9.4ms)
Completed 500 Internal Server Error in 291.7ms

ActionView::Template::Error (undefined method `first' for nil:NilClass):
    1: <%= "#{issue.tracker.name} ##{issue.id}: #{issue.subject}" %>
    2: <%= issue_url %>
    3: 
    4: <%= render_email_issue_attributes(issue, users.first) %>
    5: ----------------------------------------
    6: <%= issue.description %>
    7: 
  app/views/mailer/_issue.text.erb:4:in `_app_views_mailer__issue_text_erb___195752182905986264_52251460'
  app/models/mailer.rb:410:in `block in mail'
  app/models/mailer.rb:409:in `mail'
  app/models/mailer.rb:422:in `initialize'
  app/models/mailer.rb:445:in `method_missing'
  lib/redmine/hook.rb:61:in `block (2 levels) in call_hook'
  lib/redmine/hook.rb:61:in `each'
  lib/redmine/hook.rb:61:in `block in call_hook'
  lib/redmine/hook.rb:58:in `tap'
  lib/redmine/hook.rb:58:in `call_hook'
  lib/redmine/hook.rb:153:in `call_hook'
  app/controllers/issues_controller.rb:469:in `block in save_issue_with_child_records'
  app/controllers/issues_controller.rb:458:in `save_issue_with_child_records'
  app/controllers/issues_controller.rb:184:in `update'

```

So I did this change (I think it's correct though).