(cleanup) Check before calling property of uninitialized var

Problem:
Uninitialized variable warnings when empty inbox is opened for crunching

Fix:
- check state of variable before checking "bucket" property of email in mailcrunch inbox
  - $scope.inbox[0]

Testcase:
1)  Log into test user with empty inbox
2)  Open console
3)  Start sort session
4)  Verify no uninitialized variable warnings generated