Prevent 500 error when filter projects with push in admin area

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'fix-500-admin-projects' into 'master'

Fix 500 error when filtering projects in admin interface

Fixes #1693

See merge request !1200
Merge branch 'master' of github.com:gitlabhq/gitlabhq