Merged r14499 (#20454).

git-svn-id: http://svn.redmine.org/redmine/branches/3.0-stable@14549 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/mail_handler.rb

@@ -525,6 +525,8 @@ class MailHandler < ActionMailer::Base
 
   def find_assignee_from_keyword(keyword, issue)
     keyword = keyword.to_s.downcase
+    return nil if keyword.blank?
+
     assignable = issue.assignable_users
     assignee = nil
     assignee ||= assignable.detect {|a|