Fix pollscm backwards compatiblity

This change https://review.openstack.org/#/c/211993/ made '' an
invalid attribute value for pollscm, but that is a valid value.  We use
'' for pollscm with the option to build when a changed is pushed to
github.  This might have changed since we set it up, but you had to
specify a blank poll schedule for jenkins to build on pushes to github.
This changes allows '' as a valid value.

Change-Id: I35c45b4619584b3358562131399fc5ef16c4c5ec

jenkins_jobs/modules/triggers.py

@@ -632,7 +632,7 @@ def pollscm(parser, xml_parent, data):
         cron = data
         ipch = 'false'
 
-    if not cron:
+    if not cron and cron != '':
         raise InvalidAttributeError('cron', cron)
 
     scmtrig = XML.SubElement(xml_parent, 'hudson.triggers.SCMTrigger')

tests/triggers/fixtures/pollscm004.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<project>
+  <triggers class="vector">
+    <hudson.triggers.SCMTrigger>
+      <spec/>
+      <ignorePostCommitHooks>false</ignorePostCommitHooks>
+    </hudson.triggers.SCMTrigger>
+  </triggers>
+</project>

tests/triggers/fixtures/pollscm004.yaml

@@ -0,0 +1,3 @@
+triggers:
+  - pollscm:
+      cron: ""