handle case when trigger is none

In case when job template contains such trigger definition:

            trigger:
              github:
              pollscm:
                spec: ${_param:jenkins_pollscm_spec}

github trigger is none, but salt formulas is written so it should handle this in [1]

[1] https://gerrit.mcp.mirantis.com/gitweb?p=salt-formulas/jenkins.git;a=blob;f=jenkins/files/jobs/_common.xml;h=8b05e640f0a122d0083675df1071df5467bd21be;hb=refs/heads/master#l39

Change-Id: I3acdf6392d4349121677845fb5f6640ba760d9ae
diff --git a/jenkins/files/jobs/_common.xml b/jenkins/files/jobs/_common.xml
index 8b05e64..4dd6ab3 100644
--- a/jenkins/files/jobs/_common.xml
+++ b/jenkins/files/jobs/_common.xml
@@ -24,6 +24,7 @@
     <org.jenkinsci.plugins.workflow.job.properties.PipelineTriggersJobProperty>
       <triggers>
         {%- for type, trigger in job.get('trigger', {}).iteritems() %}
+        {%- if trigger is none %}{%- set trigger = {} %}{%- endif %}
         {%- if trigger.get('enabled', True)|to_bool %}
         {%- if type == 'reverse' %}
         <jenkins.triggers.ReverseBuildTrigger>