#4080 Fix wrong pdf format selection for invoice, when configured

To replicate this I

1) created a new pdf format at civicrm/admin/pdfFormats?reset=1 with a top margin
of about 6 metres
2) edited the invoice MessageTemplate to use this pdf format
3) enabled invoicing
4) generated a pdf from the user dashboard
5) lots of white space.....
6) reset the invoice MessageTemplate not to have a selected pdf
7) it looked ... normal