Fix NoMethodError for tags#date
What does this MR do and why?
Contributes to #454109 (closed)
Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/742521/
Problem
Apparently Gitaly can return a tagger with an empty date. This case is not handled by Rails and raises an exception.
Solution
Verify that the date object is present before returning the date.
Edited by Vasilii Iakliushin