Defect #4257 (New): Broken support for Principal plugins

I'm not sure when exactly it happens (it's worked sometime ago), but things becomes broken for Principal with empty plugin.

How to reproduce:

$ cat vendor/plugins/test_plugin/init.rb
require'redmine'require'dispatcher'Dispatcher.to_prepare:test_plugindorequire_dependency'principal'endRedmine::Plugin.register:test_plugindoend
(no other files in vendor/test_plugin)

$ rake test:units
<...>
1) Failure:
test_roles_updated(GroupTest) [/test/unit/group_test.rb:57]:
<[1, 2]> expected but was
<[1]>.
2) Failure:
test_visible_scope_for_user(IssueTest) [/test/unit/issue_test.rb:94]:
<false> is not true.
478 tests, 2036 assertions, 2 failures, 0 errors