Check for negative defined logic in conditionals

lib/ansible/playbook/conditional.py

@@ -125,9 +125,9 @@ class Conditional:
             # variable was undefined. If we happened to be
             # looking for an undefined variable, return True,
             # otherwise fail
-            if "is undefined" in original:
+            if "is undefined" in original or "is not defined" in original or "not is defined" in original:
                 return True
-            elif "is defined" in original:
+            elif "is defined" in original or "is not undefined" in original or "not is undefined" in original:
                 return False
             else:
                 raise AnsibleUndefinedVariable("error while evaluating conditional (%s): %s" % (original, e))