Anonymous user should have their icon
ApplicationHelper#avatar method returns blank string instead of
img tag for the anonymous user. The behavior causes disordered layout in IssuesController#show page.
This patch adds an icon for the anonymous user and fixes the broken layout.
The author of the issue is the anonymous user. But in the upper left corner of the page, the assignee's icon is displayed as if it is the author's icon because the anonymous user does not have their avatar icon.
No icon is displayed in the journal which was made by the anonymous user.
New anonymous icon is displayed.