Fix issue when string list settings field is null. Resolves #3683.

awx/conf/fields.py

@@ -20,8 +20,14 @@ logger = logging.getLogger('awx.conf.fields')
 
 
 class StringListField(ListField):
+
     child = CharField()
 
+    def to_representation(self, value):
+        if value is None and self.allow_null:
+            return None
+        return super(StringListField, self).to_representation(value)
+
 
 class URLField(CharField):