Fix conifg.mk render issue

For podconfig with skipTags defined but has no options cause jinja2 NonType object not iterable Error.
To fix this issue, add a NonType check after skipTags defined check to avoid NonType object iteration.

Change-Id: I7b0ca533c52acbe07899edb64a18dad75b255889
diff --git a/ansible/roles/genconfig/templates/config.mk.j2 b/ansible/roles/genconfig/templates/config.mk.j2
index c872f2c..9bfb0fa 100644
--- a/ansible/roles/genconfig/templates/config.mk.j2
+++ b/ansible/roles/genconfig/templates/config.mk.j2
@@ -41,7 +41,7 @@
 {% if build_cord_dir is defined %}
 BUILD_CORD_DIR = {{ build_cord_dir }}
 {% endif %}
-{% if skipTags is defined %}
+{% if skipTags is defined and skipTags %}
 ANSIBLE_ARGS += --skip-tags "{{ skipTags | join(",") }}"
 {% endif %}