Merged r12189 from trunk to 2.3-stable (#13632)

pdf: fix empty page attached when exporting.

Contributed by Jun NAITOH.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/branches/2.3-stable@12191 e93f8b46-1217-0410-a6f0-8f06a7374b81