Handle empty ranges for get_user_availability

When requesting a users availability with [EWSClient#get_user_availability](https://github.com/zenchild/Viewpoint/blob/57f4e02e622aaba1e475d567c0f237262e033239/lib/ews/mailbox_accessors.rb#L52) and the user is not busy during the requested time window, the current code fails with `NoMethodError: undefined method '[]' for nil:NilClass` when calling `calendar_event_array`.

The attached code will return an empty array in that case. I also wrote test cases for empty and not-empty responses.

**Please note**: This pull requests probability conflicts with https://github.com/zenchild/Viewpoint/pull/74. I'll resolve the merge conflicts in this pull request, once #74 is applied (or vice versa).