[1.2.X] Fixed #15212 -- ensure that ModelAdmin.get_actions still returns a...

[1.2.X] Fixed #15212 -- ensure that ModelAdmin.get_actions still returns a SortedDict if there are no actions. Backport of [15393].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15394 bcc190cf-cafb-0310-a4f2-bffc1f526a37