Accept None as a parameter to client.master.username

Change-Id: Ifd5a1fb38249287a49d1b18dc32d29e4d29eb03c
diff --git a/jenkins/files/_jenkins.conf b/jenkins/files/_jenkins.conf
index 3461e87..c0ac891 100644
--- a/jenkins/files/_jenkins.conf
+++ b/jenkins/files/_jenkins.conf
@@ -3,7 +3,7 @@
   url: "{{ client.master.get('proto', 'http') }}://{{ client.master.host }}:{{ client.master.port }}"
   {%- if client.master.api_key is defined %}
   api_key: {{ client.master.api_key }}
-  {%- elif client.master.username is defined %}
+  {%- elif client.master.get('username', None) %}
   user: {{ client.master.username }}
   password: {{ client.master.password }}
-  {%- endif %}
\ No newline at end of file
+  {%- endif %}