Fix that updating an issue from context menu to a none value do not set the field to blank (#33521).

Patch by Mizuki ISHIKAWA.

git-svn-id: http://svn.redmine.org/redmine/trunk@21139 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/controllers/issues_controller.rb

@@ -561,6 +561,7 @@ class IssuesController < ApplicationController
         return false
       end
     end
+    issue_attributes = replace_none_values_with_blank(issue_attributes)
     @issue.safe_attributes = issue_attributes
     @priorities = IssuePriority.active
     @allowed_statuses = @issue.new_statuses_allowed_to(User.current)