Correct delegated_host_name check

In ansible@fb50698 a check for delegated_host_name being defined was added. Make this
check safer as it breaks some playbooks.

lib/ansible/vars/__init__.py

@@ -459,7 +459,7 @@ def _get_delegated_vars(self, loader, play, task, existing_variables):
 
             templar.set_available_variables(vars_copy)
             delegated_host_name = templar.template(task.delegate_to, fail_on_undefined=False)
-            if not delegated_host_name:
+            if delegated_host_name is None:
                 raise AnsibleError(message="Undefined delegate_to host for task:", obj=task._ds)
             if delegated_host_name in delegated_host_vars:
                 # no need to repeat ourselves, as the delegate_to value