ERP5Type/patches/WSGIPublisher: Avoid boolean evaluation of foreign object.

Spotted when comparing this code with ZPublisher.Publish.
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:53

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:48