not add empty header/footer to notification emails (#13482)

Contributed by Max Horn.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11721 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/views/layouts/mailer.text.erb

@@ -1,4 +1,8 @@
+<% if Setting.emails_header.present? -%>
 <%= Setting.emails_header %>
+<% end -%>
 <%= yield %>
+<% if Setting.emails_footer.present? -%>
 -- 
 <%= Setting.emails_footer %>
+<% end -%>