Fix add to mattermost nil teams

What does this MR do?

Fixes this issue: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8320#note_21935679

For some reason @teams can be nil because list_teams is too javascript. :troll:

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?