Don't fail in syncback when a Gmail account has no labels.

Summary:
It's entirely possible for the result of GmailCrispinClient.folder_names() to
not contain a 'labels' key. Previously this could cause syncback actions to
fail with KeyError.

Test Plan:
Check that the unarchive action can now succeed for a test account
with no labels.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D582