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.