ticket: Don't offer <nil> option if already unassigned

[gitblit]

Originally reported on Google Code with ID 603

From the ticket edit page, the <nil> option is available always for responsible and
milestone, even if there is not a value currently assigned. Selecting it results in
reassigning the same value, which clutters the activity log with entries assigning
the same value.

Reported by mrjoelseph on 2015-07-01 23:19:23