Proposed fix for JENKINS-13465

JENKINS-13465 https://issues.jenkins-ci.org/browse/JENKINS-13465

Happened with Jenkins 1.506 and plugin version 0.9.1

I've not found any "parameter" field in the posted form, but not looked very deeply... May be "a" contains only a single JSONNull if formData.get("parameter") is "null" or equivalent.